Add decorator and setting to set `hints.mostly-unused` (#4243)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
Continuing the work started in
https://github.com/smithy-lang/smithy-rs/pull/4208, the new feature we
are utilizing is explained in
https://blog.rust-lang.org/inside-rust/2025/07/15/call-for-testing-hint-mostly-unused

## Description
<!--- Describe your changes in detail -->
Add a new `ManifestHintsDecorator` that allows adding a `[hints]`
section to a generated `Cargo.toml`. This also introduces a new optional
entry in `smithy-build.json`, `hintsMostlyUnusedList` that allows
indicating which crates this should be enabled for. Currently this is
only enabled for `aws-sdk-s3`, `aws-sdk-dynamodb`, and `aws-sdk-ec2`.

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
I tested both debug and release builds of the crates, with and without
the hint on an M1 Macbook Pro. You can see the full data below, but the
general outcome is that release builds are ~50% faster with the hint and
debug builds are ~40% faster. A release build of the `aws-sdk-ec2` crate
went from `124.1s` without the hint to `59.1s` with it.


<details><summary>Expand To See Test Data</summary>
<p>
All tests run after deleting the `target/` dir

## Debug No Hint
`cargo build --timings`
```
||Unit|Total|Codegen|Features|
|---|---|---|---|---|
|1.|aws-sdk-ec2 v0.0.0-local|104.8s|27.9s (27%)|default, default-https-client, rt-tokio, rustls|
|2.|aws-lc-sys v0.30.0 build script (run)|26.1s||prebuilt-nasm|
|3.|aws-sdk-s3 v0.0.0-local|21.2s|5.2s (24%)|default, default-https-client, rt-tokio, rustls, sigv4a|
|4.|aws-sdk-dynamodb v0.0.0-local|13.7s|4.0s (29%)|default, default-https-client, rt-tokio, rustls|
```

| Unit             | Total  | Codegen     |
| ---------------- | ------ | ----------- |
| aws-sdk-ec2      | 104.8s | 27.9s (27%) |
| aws-sdk-s3       | 21.2s  | 5.2s (24%)  |
| aws-sdk-dynamodb | 13.7s  | 4.0s (29%)  |

## Debug Hint
`cargo +nightly -Zprofile-hint-mostly-unused build --timings`

```
||Unit|Total|Codegen|Features|
|---|---|---|---|---|
|1.|aws-sdk-ec2 v0.0.0-local|66.4s|2.4s (4%)|default, default-https-client, rt-tokio, rustls|
|2.|aws-lc-sys v0.30.0 build script (run)|25.0s||prebuilt-nasm|
|3.|aws-sdk-s3 v0.0.0-local|14.6s|0.8s (5%)|default, default-https-client, rt-tokio, rustls, sigv4a|
|4.|aws-sdk-dynamodb v0.0.0-local|9.1s|0.4s (4%)|default, default-https-client, rt-tokio, rustls|
```

| Unit             | Total | Codegen   |
| ---------------- | ----- | --------- |
| aws-sdk-ec2      | 66.4s | 2.4s (4%) |
| aws-sdk-s3       | 14.6s | 0.8s (5%) |
| aws-sdk-dynamodb | 9.1s  | 0.4s (4%) |


## Release No Hint
`cargo build --timings --release `

```
||Unit|Total|Codegen|Features|
|---|---|---|---|---|
|1.|aws-sdk-ec2 v0.0.0-local|124.1s|54.3s (44%)|default, default-https-client, rt-tokio, rustls|
|2.|aws-lc-sys v0.30.0 build script (run)|34.3s||prebuilt-nasm|
|3.|aws-sdk-s3 v0.0.0-local|28.5s|11.4s (40%)|default, default-https-client, rt-tokio, rustls, sigv4a|
|4.|aws-sdk-dynamodb v0.0.0-local|19.3s|8.5s (44%)|default, default-https-client, rt-tokio, rustls|
```

| Unit             | Total  | Codegen     |
| ---------------- | ------ | ----------- |
| aws-sdk-ec2      | 124.1s | 54.3s (44%) |
| aws-sdk-s3       | 28.5s  | 11.4s (40%) |
| aws-sdk-dynamodb | 19.3s  | 8.5s (44%)  |


## Release Hint
`cargo +nightly -Zprofile-hint-mostly-unused build --timings --release`

```
||Unit|Total|Codegen|Features|
|---|---|---|---|---|
|1.|aws-sdk-ec2 v0.0.0-local|59.1s|1.0s (2%)|default, default-https-client, rt-tokio, rustls|
|2.|aws-lc-sys v0.30.0 build script (run)|34.4s||prebuilt-nasm|
|3.|aws-sdk-s3 v0.0.0-local|14.7s|0.4s (3%)|default, default-https-client, rt-tokio, rustls, sigv4a|
|4.|tokio v1.47.1|11.1s|7.3s (65%)|bytes, default, fs, full, io-std, io-util, libc, macros, mio, net, parking_lot, process, rt, rt-multi-thread, signal, signal-hook-registry, socket2, sync, time, tokio-macros|
|5.|aws-sdk-dynamodb v0.0.0-local|10.4s|0.4s (4%)|default, default-https-client, rt-tokio, rustls|
```

| Unit             | Total | Codegen   |
| ---------------- | ----- | --------- |
| aws-sdk-ec2      | 59.1s | 1.0s (2%) |
| aws-sdk-s3       | 14.7s | 0.4s (3%) |
| aws-sdk-dynamodb | 10.4s | 0.4s (4%) |

</p>
</details> 

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

----

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