Prompt user to choose NABSL when more than one exists

[coderabbitai]

Summary

When a user has more than one NonAdminBackupStorageLocation (NABSL) in their namespace (approved or not approved), the CLI should prompt the user to choose which storage location to use for nonadmin backup creation, rather than failing or requiring manual flag input.

Background

PR #193 (#193) introduced auto-selection of the storage location when exactly one NABSL exists in the namespace. This follow-up issue tracks the complementary behavior: interactive prompting when multiple NABSLs exist.

Desired Behavior

• If exactly one NABSL exists → auto-select it (already implemented in Auto select when only one NABSL exists #193)
• If more than one NABSL exists (regardless of approved/not approved status) → prompt the user interactively to pick one before proceeding with backup creation

References

• PR Auto select when only one NABSL exists #193: Auto select when only one NABSL exists #193
• Requested by @kaovilai, noted by @weshayutin