sql,server: abstract at least 3 of the tenant rangefeed watchers

Requested by @yuzefovich in #110947.

**Is your feature request related to a problem? Please describe.**

The code in the following watchers is very similar:
- tenant settings watcher
- tenant info watcher
- tenant capabilities watcher
- settings watcher

It is very redundant and yet needs to do things in a precise order to avoid race conditions and deadlocks.
We need to reduce this complexity to increase maintainability.

**Describe the solution you'd like**

Abstract at least 2 (preferably at least 3) of the watchers into a common library used in multiple places.

Also merge the tenant capabilities and info watchers into just 1.

Epic: CRDB-26691

Jira issue: CRDB-31697