Implement the remaining part of supporting operation level configuration (#2814)

## Motivation and Context
Implements the actual meat of `config_override` introduced [as a
skeleton in the past](https://github.com/awslabs/smithy-rs/pull/2589).

## Description
This PR enables operation-level config via `config_override` on a
customizable operation (see
[config-override.rs](https://github.com/awslabs/smithy-rs/blob/8105dd46b43854e7909aed82c85223414bc85df5/aws/sdk/integration-tests/s3/tests/config-override.rs)
integration tests for example code snippets).

The way it's implemented is through `ConfigOverrideRuntimePlugin`. The
plugin holds onto two things: a `Builder` passed to `config_override`
and a `FrozenLayer` derived from a service config (the latter is
primarily for retrieving default values understood by a service config).
The plugin then implements the `RuntimePlugin` trait to generate its own
`FrozenLayer` that contains operation-level orchestrator components.
That `FrozenLayer` will then be added to a config bag later in the
orchestrator execution in a way that it takes precedence over the
client-level configuration (see
[register_default_runtime_plugins](https://github.com/awslabs/smithy-rs/blob/8105dd46b43854e7909aed82c85223414bc85df5/codegen-client/src/main/kotlin/software/amazon/smithy/rust/codegen/client/smithy/generators/OperationGenerator.kt#L65-L71)).

A couple of things to note:
- The size of `ConfigOverrideRuntimePlugin::config` code-generated is
getting large. Maybe each customization defines a helper function
instead of inlining logic directly in `config` and we let the `config`
method call those generated helpers.
- The PR does not handle a case where `retry_partition` within
`config_override` since it's currently `#[doc(hidden)]`, e.g.
```
client
    .some_operation()
    .customize()
    .await
    .unwrap()
    .config_override(Config::builder().retry_partition(/* ... */))
    ...
```

## Testing
- Added tests in Kotlin
[ConfigOverrideRuntimePluginGeneratorTest.kt](https://github.com/awslabs/smithy-rs/pull/2814/files#diff-04a76a43c2adb3a2ee37443157c68ec6dad77fab2484722b370a7ba14cf02086)
and
[CredentialCacheConfigTest.kt](https://github.com/awslabs/smithy-rs/pull/2814/files#diff-32246072688cd11391fa10cd9cb38a80ed88b587e95037225dbe9f1a482ebc5d).
~~These tests are minimal in that they only check if the appropriate
orchestrator components are inserted into a config override layer when a
user calls a certain builder method as part of `config_override`.~~
- Added integration tests
[config-override.rs](https://github.com/awslabs/smithy-rs/pull/2814/files#diff-6fd7a1e70b1c3fa3e9c747925f3fc7a6ce0f7b801bd6bc46c54eec44150f5158

)

----

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choice._

---------

Co-authored-by: Yuki Saito <awsaito@amazon.com>