multitenant: ability to set default tenant capabilities

Currently, tenant capabilities are granted on a per-tenant basis using the `ALTER TENANT ... GRANT CAPABILITY` syntax.

However, we provide no way for the user to grant a capability to all current and future tenants. Tenant settings allow for all-tenant overrides that apply to all tenants (provided they don't have a tenant-specific) override. A similar feature for tenant capabilities would allow the operator to similarly express the "default" set of capabilities they would like to grant tenants.

You can create a "template" tenant with the set of capabilities you would like by default and use the `CREATE TENANT .. LIKE` syntax, but the capabilities are applied to the tenant only at creation time and thus cannot be updated en-masse later.

Jira issue: CRDB-32780