feat: add typed message tags

[bjarn]

Summary

• Add the immutable MessageTag model.
• Add a typed tags(MessageTag...) overload.
• Keep the existing tags(Map...) overload.
• Keep tag() for the legacy single tag.

Validation

• Validate each MessageTag when it is created.
• Reject reserved names and duplicate case-sensitive names.
• Allow 20 total tags, including the legacy single tag.

Documentation

• Replace the outdated tag example with typed structured tags.
• Document compatibility with the previous map input.

Verification

• ./gradlew test --no-daemon
• GitHub test matrix for Java 8, 11, 17, and 21
• GitHub checkstyle check