PR#1778 feat: added PooledThreadExecutor::WaitUntilStopped function + updates

[SergeyRyabinin]

Issue #, if available:
Updating the PR for #1778
Description of changes:

1. Add proposed method to the base interface class
2. Implement in a default executor
3. Split Executor uber-files into specific headers and translation units
   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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[sbiscigl]

note that cspell is failing for

203/393 ./src/aws-cpp-sdk-core/include/aws/core/utils/threading/PooledThreadExecutor.h 6.29ms X
/Users/sbiscigl/aws-sdk-cpp/src/aws-cpp-sdk-core/include/aws/core/utils/threading/PooledThreadExecutor.h:50:38 - Unknown word (threadpool)

if you could add that to the cspell word check in this PR