workaround fork for CRT

[TingDaoK]

Notes:

• A prove of concept, not ready to merge yet.
• Needs adding integration tests and polish the code
  • I guess boto3 try to avoid using awscrt directly, I guess we could have s3transfer to expose join_all_native_threads
  • suggestions are welcome, or feel free to starting new branch if it's easier.
• It depends on Fork workaround awslabs/aws-crt-python#628, so CI will fail until we update the dependency.

Issue:

• In CRT, we creates background threads, and fork is not safe with background threads at all.
• From CRT perspective, it's basically impossible to track all threads and recreate them after fork. So, pushing the threads management to boto3 here.
• As boto3 owns the CRT S3 client, which is the only CRT resource boto3 uses with background threads (I am not 100% percent sure, but looked into botocore, the CRT singer depends on static credentials provider, so it's not creating background threads, and it works on my reproduce code.)
• So, boto3 can register a fork handler to clean up the CRT client and wait for threads to join.
• From the forked process, new client will be created if needed, and it will be safe to use.
• Notes: logger can create background threads, but it's not managed thread that will block the call, and it's also kind safe to be used (not lead to a crash at least)