Reduce configure times when regenerating clients

[apache-hb]

Description of changes:

Runs clang-format in parallel after generating client code. On my epyc 9654 this reduces the time to configure a new build directory by about ~75%. The implementation is a bit naive and only splits the generated file set into equal sizes, when regenerating a single client some clang-format processess will only format a single file.

# build.sh
cmake -S . -B $BUILDDIR \
    -DAUTORUN_UNIT_TESTS=OFF \
    -DREGENERATE_CLIENTS=ON \
    -DCMAKE_BUILD_TYPE=MinSizeRel \
    -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
    -DCMAKE_INSTALL_PREFIX=$INSTALLDIR \
    -GNinja

elliothb@ELLIOT-SERVER:~/github/aws-sdk-cpp-parallel-clang-format$ time ./build.sh build-before $(pwd)/install-before > before.log 2>&1

real    12m16.217s
user    143m26.334s
sys     15m36.796s
elliothb@ELLIOT-SERVER:~/github/aws-sdk-cpp-parallel-clang-format$ time ./build.sh build-after $(pwd)/install-after > after.log 2>&1

real    2m45.729s
user    141m31.590s
sys     16m12.126s

I think the only mild downside is the logs look a bit uglier as they now contain interleaved invocations of clang-format

Clang-formating 782 files
Clang-formating 782 files
⚠️  clang-format is already on your PATH and installed at
    /usr/local/bin/clang-format. Downloading and running anyway.
Clang-formating 782 files
⚠️  clang-format is already on your PATH and installed at
    /usr/local/bin/clang-format. Downloading and running anyway.
⚠️  clang-format is already on your PATH and installed at
    /usr/local/bin/clang-format. Downloading and running anyway.
Clang-formating 782 files
Clang-formating 782 files
⚠️  clang-format is already on your PATH and installed at
    /usr/local/bin/clang-format. Downloading and running anyway.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

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[kai-ion]

@apache-hb Thanks for bringing this PR! It looks good, i'm going to run it through our CI and once it passes I will merge the PR

[kai-ion]

@apache-hb Hey, I ran it through our CI and im seeing an Error while executing command: git gc. Reason: exit status 128

[apache-hb]

I think that git gc error is transient, I don't think my git commits are causing it.

[kai-ion]

@apache-hb There is a git gc timing issue. Git's background auto-gc gets triggered during codegen.  With the new parallel formatting, the build phase completes while auto-gc is still running — its gc.pid lock file gets captured in the artifact zip. When PrepareArtifacts unpacks the zip and runs git gc, it sees the stale lock and fails. We have a fix out for it in release next week.

[kai-ion]

@apache-hb PR has been merged in commit