Add "az spring flush-virtualnetwork-dns-settings" feature

[hanren2021]

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

Related command

az spring update

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.
  Additional command for flush virtual network DNS settings for Azure Spring Apps

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.
You only need to update the version information in file setup.py and historical information in file HISTORY.rst in your PR but do not modify src/index.json.

[azure-client-tools-bot-prd]

⚠️Azure CLI Extensions Breaking Change Test

⚠️spring

rule	cmd_name	rule_message	suggest_message
⚠️ 1001 - CmdAdd	spring flush-virtualnetwork-dns-settings	cmd spring flush-virtualnetwork-dns-settings added

[azure-client-tools-bot-prd]

Hi @hanren2021,
Please write the description of changes which can be perceived by customers into HISTORY.rst.
If you want to release a new extension version, please update the version in setup.py as well.

[yonzhan]

Thank you for your contribution! We will review the pull request and get back to you soon.

[azclibot]

[Release] Update index.json for extension [ spring ] : https://dev.azure.com/azclitools/internal/_build/results?buildId=85551&view=results