As discussed with @stevendanna @dt and @ajwerner :

• we want a setting class which uses a single shared value between system and secondary tenants.
• the current behavior of TenantReadOnly (one value per tenant) can be readily emulated by a TenantReadWrite setting + an OVERRIDE in the system tenant.

So to achieve point 1, we can redefine TenantReadOnly to make settings take their value from the system tenant.

Informs #77472

Epic: CRDB-6671

Jira issue: CRDB-18406