The provider exposed mirrorlist but not mirror_strategy, so the API's
mirror load-balancing strategy (round_robin/least_conn) was unreachable
via Terraform. Add mirror_strategy the same way mirrorlist was added,
scoped to rpm/deb/alpine remotes.
- schema: optional string mirror_strategy on the remote resource
- ValidateConfig: reject on non-rpm/deb/alpine types and validate the
round_robin/least_conn enum at plan time
- wire model + modelToAPI/apiToModel (empty->null to avoid perpetual diff)
- datasource (computed) + README + rpm example
- unit tests: modelToAPI/apiToModel round-trip and ValidateConfig matrix
The API returns null for empty arrays, but OpenTofu requires that the
state match the plan exactly — an empty list [] in the plan must remain
[] in the state, not become null. This caused "inconsistent result after
apply" errors on every resource with empty optional list fields like
mutable_patterns and ban_tags.