[Spring] Add subcommand "az spring app connect".

[LGDoor]

---

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

Related command

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 pull request is merged into main branch, a new pull request will be created to update src/index.json automatically.
The precondition is to put your code inside this repository and upgrade the version in the pull request but do not modify src/index.json.

[yonzhan]

Spring

[zhoxing-ms]

Could you add some tests for those new commands?

[zhoxing-ms]

Please add the description of this PR in the HISTORY.rst. If you want to release a new extension version for this PR, please also upgrade the api-version defined in setup.py

[azclibot]

[Release] Update index.json for extension [ spring ] : https://dev.azure.com/azure-sdk/internal/_build/results?buildId=1867410&view=results