Allow for Non-MR System Database in MR Serverless Hosts

Serverless creates single region clusters in the multi-region Serverless host. Currently, we configure the system database of single region tenants in the multi-region host as a multi-region database because its the only way to constrain which regions the tenant can see. We need to somehow extend the region provider so we can restrict regions without configuring the system database as MR:
https://github.com/cockroachdb/cockroach/blob/b4c58a014a82dbc6e193c2633fc0691f872928b0/pkg/sql/regions/region_provider.go#L74

There are few issues caused by giving single region tenants a multi-region system db:
1. It breaks PCR of single-region clusters between different regions. See https://github.com/cockroachdb/cockroach/issues/119934 for more details.
2. It configures descriptors as a global table. This slows down schema changes and does not provide a benefit in single region clusters.

Jira issue: CRDB-42547

Epic CRDB-40070