[#12125] design-docs: Add tag assignment values design

[roryqi]

What changes were proposed in this pull request?

Add a design document for tag assignment values.

The design covers:

• multiple values per tag assignment
• allowedValues on tag creation
• one row per assignment value in tag_relation_meta
• pair-array assignment updates through tagsToAdd and tagsToRemove
• exact value lookup through GET /tags/{tag}/objects?value=...

Why are the changes needed?

Tag assignments are currently boolean and cannot carry object-specific values. This design defines the API, storage, client, cache, authorization, event, and test plan for assignment-level tag values.

Fix: #12125

Does this PR introduce any user-facing change?

No code or API behavior changes. This PR only adds a design document.

How was this patch tested?

• git diff --check apache/main...HEAD
• Checked the design document for stale standalone search API references and placeholders.

[github-actions]

Code Coverage Report

Overall Project	67.77%	🟢
Files changed	No Java source files changed	-

Module	Coverage
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