[RecoveryServices.Backup]Trigger AFS container/item discovery only when needed

[hiaga]

Description

• Now Discover protection containers only when it's not already registered/discovered.
• Trigger item inquiry only if it is not already discovered.

Checklist

• [Yes] I have read the Submitting Changes section of CONTRIBUTING.md
• [Yes] The title of the PR is clear and informative
• [Yes] The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• [Yes] The PR does not introduce breaking changes
• [N/A] If applicable, the changes made in the PR have proper test coverage
• [N/A] For public API changes to cmdlets:
  • [N/A] a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
  • [N/A] the markdown help files have been regenerated using the commands listed here