fix(cas-backend): Rotate credentials does not remove default flag

[javirln]

This pull request refactors how the CLI handles updates to CAS Backend resources, specifically for the description and default fields. The main improvement is to only update these fields if the user explicitly sets the corresponding flags, preventing accidental overwrites with zero values.

CLI update logic improvements:

• Added package-level pointer variables (isDefaultCASBackendUpdateOption, descriptionCASBackendUpdateOption) and helper functions (captureUpdateFlags, handleDefaultUpdateConfirmation) to ensure updates only occur when flags are explicitly set, and to centralize confirmation prompts for default backend changes. [1] [2]
• Refactored update commands for all backend types (azure-blob, inline, oci, aws-s3) to use the new flag capture and confirmation logic, and to pass pointer values to update options. [1] [2] [3] [4]

Internal types and API updates:

• Changed NewCASBackendUpdateOpts struct fields Description and Default to pointers, so unset fields are distinguishable from zero values.
• Updated the CASBackendServiceUpdateRequest protobuf struct and its getter methods to use pointer types for Description and Default, supporting the new semantics in API requests. [1] [2] [3]