Allow benchmark test to be build with CRT Client

[sbiscigl]

Description of changes:

Defaults the benchmark tests to use CRT client, can be optionally turned off. ie.

Build benchmarks with CRT client:
-DBUILD_ONLY="s3;s3-crt;monitoring" -DBUILD_BENCHMARKS=ON -DAWS_SDK_BENCHMARK_USE_CRT=ON

Build benchmarks with vanilla client:
-DBUILD_ONLY="s3;s3-crt;monitoring" -DBUILD_BENCHMARKS=ON -DAWS_SDK_BENCHMARK_USE_CRT=OFF

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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