Add "Spring" extension and deprecate "Spring-Cloud" extension

[yuwzho]

---

This checklist is used to make sure that common guidelines for a pull request are followed.

• Add new extension "spring" and mark the old extension "spring-cloud" as deprecated.
  

• The top-level command "spring-cloud" is hided, user can no longer see it when typing "az -h"

  

• The sub-level commands under "spring-cloud" is not hided.
  

Related command

az spring
az spring-cloud

General Guidelines

• Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• Have you run python scripts/ci/test_index.py -q locally?

For new extensions:

• My extension description/summary conforms to the Extension Summary Guidelines.

About Extension Publish

There is a pipeline to automatically build, upload and publish extension wheels.
Once your PR is merged into master branch, a new PR will be created to update src/index.json automatically.
The precondition is to put your code inside this repo and upgrade the version in the PR but do not modify src/index.json.

[yonzhan]

Spring

[zhoxing-ms]

You may consider adding the public document link for spring migration to the help description of az spring-cloud and the message_func of deprecate_info.
Because the public document can help users better understand the context and background of this migration.