refactor: 💡 Use validator to validate or default deployment

[yuwzho]

In most secenarios under az spring-cloud app, it can accept a
parameter deployment. If the deployment is set, the command will update apps/deployments resource. Otherwise, it will look for one deployment under the app meets deployment.properties.active = true to be operated.
In today's code, each command handle such logic to fetch or use default value. This PR abstract this logic to validator, and make custom command can be focus on the operation logic

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[zhoxing-ms]

LGTM. By the way, may I ask do we need to write the description of this change into HISTORY.rst and update the setup.py to release a new version

[yuwzho]

LGTM. By the way, may I ask do we need to write the description of this change into HISTORY.rst and update the setup.py to release a new version

No, it is not a fix or feature. Will be shipped with coming features.