feat: add typed message tags

[bjarn]

Summary

• Add the MessageTag value object for reusable name and value tags.
• Accept MessageTag objects and the existing array input in EmailEndpoint::tags().
• Keep EmailEndpoint::tag() for the legacy single tag.
• Serialize both input forms to the same Sending API wire format.

Validation

• Enforce the documented name and value formats.
• Reject reserved __lettermint names without regard to case.
• Reject duplicate case-sensitive names.
• Allow 20 total tags, including the legacy single tag.

Documentation

• Add typed tag usage to the README.

Verification

• composer test
• composer analyse