ICU-23310 Add U_LIFETIME_BOUND annotations for unistr.h et al.

[roubert]

Checklist

• Required: Issue filed: ICU-23310
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable
• Approver: Feel free to merge on my behalf

ALLOW_MANY_COMMITS=true

[eggrobin]

Looks good.
Feel free to do add the additional utext_open bounds in a subsequent PR if that’s more convenient.

[roubert]

I'll keep those comments unresolved here and go back to them later.