Add global thread pool concept to azure-core

[alzimmermsft]

Description

Adds a global thread pool concept to azure-core so that all SDKs don't need to manage their own thread pools, possibly leading to hundreds or thousands of threads.

This also includes a concept where the global thread pool can be configuration. The reason for this is cases where the Threads created by the pool need specific permissions, such as environments where SecurityManager is being used. We can't handle this centrally as we don't know much about the environment using the SDK, and allowing the thread pool to be set would allow for the environment to use an appropriate way to create threads. Where permissions are properly granted, or additional permissions for the Azure SDK created threads aren't needed. Cut from PR

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
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• Pull request includes test coverage for the included changes.

[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

com.azure:azure-core

[srnagar]

LGTM