Add Token::into_encoded

[asmello]

Summary

• Adds Token::into_encoded(self) -> Cow<'a, str>, which consumes the Token and returns its encoded string representation.
• Since the Token internal representation is already RFC 6901 encoded, this is a plain move of the inner Cow -- no allocation.

Closes #115.

Test plan

• cargo test --lib token
• cargo test --doc token
• cargo clippy --all-features

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.6%. Comparing base (0018d9e) to head (144fff7).

Additional details and impacted files

Files with missing lines	Coverage Δ
src/token.rs	98.3% <100.0%> (+<0.1%)	⬆️

... and 1 file with indirect coverage changes

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