[NEW]mgmt resources, sync implementation for TaskGroup

[XiaofeiCao]

About this PR

This PR is a copy of previous PR with commits squashed.
Last one fails strangely for revapi check: #45068 (comment)
One of the commit added TaskGroup.failedFuture with public visibility, and the next one changed it to private. Shouldn't trigger any breaking changes to 2.50.0... Not sure how this bug happens..

Description

Part of sync-stack support for mgmt premium, and core-v2 if we want to keep premium packages.

Goal

1. Provide sync interfaces for TaskGroup invocation without Reactor..
2. Support specifying Executor to achieve task execution parallelism.

Context

A rough flow of TaskGroup.invokeAsync:

About this PR

TaskGroup.invoke's major change is in step4 -> Concurrently execute tasks.
Instead of Flux.merge, use CompletableFuture.allOf to join all CompletableFutures kicked off by CompletableFuture.supplyAsync with default or specified Executor.

Specify Executor by setting syncExecutor in InvocationContext, and execute via TaskGroup.invoke(InvocationContext).

To run task in the same thread as TaskGroup.invoke, set syncExecutor to Runnable::run.

Default Executor(if not specified) is ForkJoinPool.commonPool(), which is also the default Executor for CompletableFuture.

Test

Added tests in the same package as async ones, with Sync prefix.

Future works

1. New sync APIs in resource base models, with Reactor as its default implementation.
2. Override these new sync Resource API implementations for all packages.
3. Remove Reactor default implementations for sync APIs.

Please add an informative description that covers that changes made by the pull request and link all relevant issues.

If an SDK is being regenerated based on a new swagger spec, a link to the pull request containing these swagger spec changes has been included above.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[azure-sdk]

API Change Check

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

com.azure.resourcemanager:azure-resourcemanager-resources

[azure-sdk]

API change check

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

com.azure.resourcemanager:azure-resourcemanager-resources

[XiaofeiCao]

/azp run java - pullrequest

[azure-pipelines]

Azure Pipelines failed to run 1 pipeline(s).

[XiaofeiCao]

pullrequest CI is having issue triggering...
Possibly caused by: #45150 (comment)
Last one succeeded: https://dev.azure.com/azure-sdk/public/_build/results?buildId=4805872&view=results