Allow common storage prefix subpaths

[zazulam]

What this PR does

Allows storage prefixes to use common bucket subpath patterns with /, -, and _, such as team-a/mimir_blocks. Leading, trailing, repeated, and overlapping parent/child prefixes remain invalid.

Which issue(s) this PR fixes or relates to

N/A

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated.
• about-versioning.md is not required; this is not an experimental feature.

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