{spring-cloud} Add spring-cloud create --sku Enterprise

[yuwzho]

---

Description

This PR adds az spring-cloud create --sku Enterprise choice. Since when sku=Enterprise, the creation steps are very different with others (later PR will introduce some other parameters which can be used in Enterprise only), this PR adds a router before custom.py for create command, routing the command whose sku==Enterprise to another impl file. In the new impl file, there will be some steps like creating the top level resource, later creation nested resource for enterprise only. All tiers shares the same step to create top level resource, the new impl file invokes the top level resource creation impl from custom.py as well.

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

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-cloud

[wangzelin007]

Please write the description of changes into HISTORY.rst and update setup.py to release the new extension version.

[yuwzho]

Please write the description of changes into HISTORY.rst and update setup.py to release the new extension version.

No, we will announce the public preview on Jan. 17th, for the public preview, we have a lot of new features contributed to CLI. Will update the release notes and version together after all changes merged.

[wangzelin007]

LGTM

[yuwzho]

#4294