ICU-22097 Use string_view constants for option names and formatter names

[catamorphism]

This PR implements most of the suggested changes from #3216 that weren't handled before merging.

Checklist

• Required: Issue filed: ICU-22097
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 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