Avoid unnecessary Cargo crate rebuilds

This commit modifies the Gradle buildscripts to avoid unnecessary Cargo
rebuilds of the generated crates, decreasing development iteration
cycles.

Prior to this commit, if you ran a crate-generating command _twice_ on
any of the `codegen-test`, `codegen-server-test`, `sdk-codegen-test`,
and `codegen-server-test:python` subprojects (without making any changes
to the codegen code), both invocations would take the same time: Cargo
would recompile the crate and its dependencies, even when the generated
crate is identical. This is because Gradle deletes everything under
`buildDir` when running the `generateSmithyBuild` task:

```
> Task :codegen-server-test:smithyBuildJar
Deleting stale output file: /local/home/davidpz/workplace/smithy-ws/src/SmithyRsSource/codegen-server-test/build/smithyprojections/codegen-server-test
```

So the files get recreated each time.

While developing, it is likely that only a small number of the generated
crate files are modified across rebuilds. [Cargo uses
`mtime`](https://github.com/rust-lang/cargo/issues/6529) (among other
factors) to determine whether it needs to recompile a unit. Indeed,
running with `CARGO_LOG=cargo::core::compiler::fingerprint=trace` yields
`err: current filesystem status shows we're outdated`. This commit adds
a Gradle task that compares the hashes of the newly generated files with
the (previously cached) old ones, and restores their `mtime`s if the
hashes coincide.

Another issue was causing unnecessary crate rebuilds. Prior to this
commit, we were sending `RUSTFLAGS=-D warnings` when invoking Cargo.
However, a common thing to do after generating a crate is to open its
contents in an editor. The editor's `rust-analyzer` would compile the
crate and its dependencies without the `RUSTFLAGS` we had used earlier.
The next time you rebuilt the crate, Cargo would claim `err: RUSTFLAGS
has changed: previously [], now ["-D", "warnings"]` and recompile
everything again. This commit refactors the Gradle tasks so as to not
send these flags when invoking Cargo, instead generating a
`.cargo/config.toml` containing these flags. This way, `rust-analyzer`
also picks them up and does not need to recompile the crates.

With both patches, Cargo avoids unnecessary crate rebuilds. All in all,
the second invocation of a `./gradlew --info -P modules='simple' -P
cargoCommands='test' codegen-server-test:build` command now takes 15
seconds less than the first invocation on my `c5.4xlarge` machine; Cargo
does not need to do _any_ work on the second invocation.

This commit also refactors the `build.gradle.kts` files of the `sdk`,
`sdk-codegen-test`, `codegen-test`, `codegen-server-test`, and
`codegen-server-test:python` subprojects to make them DRYer and more
consistent. The last 4 subprojects' buildscripts are now much shorter,
with all the common logic having been moved to `CodegenTestCommon.kt`.
Note that we have made the last 4 subprojects' `cargo check` and `cargo
doc` invocations use the same set of flags than in the `sdk` subproject
for consistency.

Closes #1378.
Closes #1412.