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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