Validation for pgBackRest Data Sources

[andrewlecuyer]

Adds validation to ensure only cloud-based repos (S3, Azure or GCS) are configured as a pgBackRest data source.

Checklist:

• Have you added an explanation of what your changes do and why you'd like them to be included?
• Have you updated or added documentation for the change, as applicable?
• Have you tested your changes on all related environments with successful results, as applicable?
  • Have you added automated tests?

Type of Changes:

• New feature
• Bug fix
• Documentation
• Testing enhancement
• Other

What is the current behavior (link to any open issues here)?

Volume-based repos can be specified as a pgBackRest data source, even though they are not allowed.

What is the new behavior (if this is a feature change)?

• Breaking change (fix or feature that would cause existing functionality to change)

Only cloud-based repos (S3, Azure or GCS) can be configured as a pgBackRest data source.

Other Information: