cluster: add code to keep track of which server each settings object is attached to

In #112395 we found that many existing tests inadvertently share the same `Settings` object between all the servers in a test cluster. This can cause some hard-to-debug test failures, but more importantly might obscure some actual defects - all servers have their settings state perfectly in sync which is not the case in a real cluster.

This issue tracks adding a general test-only mechanism that keeps track whether a `Settings` object is "attached" to a server and panics if we try to attach it twice.

Jira issue: CRDB-32425