feat: add typed message tags

[bjarn]

Summary

• Add the MessageTag request model and constructor.
• Add typed tags to direct, batch, and fluent email sends.
• Keep tag() for the legacy single tag.
• Add a dedicated validation error.

Validation

• Validate names and values without trimming or normalization.
• Reject reserved names and duplicate case-sensitive names.
• Allow 20 total tags, including the legacy single tag.

Documentation

• Add typed structured tag usage to the README.

CI maintenance

• Update the declared stable Rust version to 1.98.
• Update quinn-proto to 0.11.15 to resolve the reported security advisory.

Verification

• cargo fmt --check
• cargo test
• cargo audit
• cargo build