fix EitherString::as_cow implementation

[davidhewitt]

Change Summary

• Unnecessary .clone() in EitherString::as_cow could clone strings needlessly
• Split the lifetimes in EitherString, already done in EitherBytes and describes the possible data flow better

Related issue number

N/A

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1794 will not alter performance

_{Comparing dh/either-string-lifetimes (9c8e257) with main (d0dbb09)}

Summary

✅ 163 untouched