Add picklist ADMIN_AVAILABLE_UNITS_FOR_ENROLLMENT for AC unit changes

[gigxz]

Merging this PR

• use the squash-merge strategy for PRs targeting a release-X branch
• use a merge-commit or rebase strategy for PRs targeting the stable branch

Description

Issue: https://github.com/open-path/Green-River/issues/7236

• Add a new picklist for units available for an enrollment. unlike the current AVAILABLE_UNITS_FOR_ENROLLMENT picklist, it also includes units with different unit types. This is quite customer specific, it is needed for a new Occurrence Point form they will use to allow admins to change unit type, which will cut down on support burden.

Opened against #5022 so I could test the features together, but it's not actually dependent

Type of change

new feature

Checklist before requesting review

• I have performed a self-review of my code
• I have run the code that is being changed under ideal conditions, and it doesn't fail
• If adding a new endpoint / exposing data in a new way, I have:
• Any major architectural changes are supported by an approved ADR (Architectural Decision Record)
• I have updated the documentation (or not applicable)
• I have added spec tests (or not applicable)
• I have provided testing instructions in this PR or the related issue (or not applicable)

[ttoomey]

Looks good!