settings: tenant-ro should read host's value

[dt]

Tenant ReadOnly settings should observe the host's value if they are not overridden. This is similar to a behavior described in the RFC as "SystemVisible" (#79223) where a tenant may wish to adjust some behavior depending on how the host cluster is configured but does not control that configuration, such as send batches according to its size limits or enable changefeeds if it has rangefeeds enabled, etc.

There is little benefit to the current behavior of a non-overridden setting returning just default value: we can already get that behavior from a tenant-rw setting, including preventing it from being set by simply adding an override on the host side. The ability to observe but not set the host's value is however quite distinct from the tenant-rw behavior, and would be valuable in the separate tenant-ro class.

Jira issue: CRDB-16756

[stevendanna]

I'm closing this as I think we got to a place where we've solved the underlying use cases here.