SNOW-2111927: fix thread handle leak from event loop

[sfc-gh-ext-simba-hx]

teamwork issue 1265
The handle leaks are from two locations:

• from ClientBootstrap which could be fixed by the workaround here
• from EventLoopGroup which has to be fixed in aws sdk.

This PR is for fixing handle leak from EventLoopGroup

[Copilot]

Pull Request Overview

This PR addresses a handle leak in the EventLoopGroup by ensuring threads are managed rather than detached.

• Sets the thread join strategy to AWS_TJS_MANAGED to allow proper cleanup.
• Adds a comment indicating the change for SNOW-2111927.

Comments suppressed due to low confidence (1)

crt/aws-crt-cpp/crt/aws-c-io/source/event_loop.c:124

• Add or update a unit test to verify that using AWS_TJS_MANAGED indeed prevents thread handle leaks when the event loop group is shut down.

            thread_options.join_strategy = AWS_TJS_MANAGED;