[BE] Fallback resource metadata for assignable permission groups

Ref: !215349 (comment 2949797008)

Currently, permission groups relies on _metadata.yml of resource dirs in authz/permissions/ (validated to have a _metadata.yml) as long as the their resource matches that of an individual permission's.

However, a permission group can combine permissions from different resources. For example, a work item group might include both epic and issue permissions. In that case, the group's resource dir might not match a resource dir in authz/permissions/ so it won't have a _metadata.yml it can reference.

TODO

Handle cases where a permission group doesn't fall under a <resource>/ dir that exists in authz/permissions/.

Implementation plan

Add fallback _metadata.yml for permission group resource dirs that don't match a resource dir in authz/permissions/.