server: tenant SQL servers lack admission control

In testing patches to integrate elastic CPU pacing into the SQL-level code for various bulk operations like producing and compressing SSTs of key-value pairs for ingestion during index backfill or IMPORT, I discovered the pacer factory in all non-system-tenant SQL servers is uninitialized and returns noop pacers.

As we continue to invest in integrating pacing into CPU-intensive SQL-level logic we should reconfigure tenant servers to have real AC infra in place for use by this pacing, to ensure it works for UA, Standard and Basic customers as well.

Internal Slack thread: https://cockroachlabs.slack.com/archives/C02HWA24541/p1753136420170029

relevant TODO:
https://github.com/cockroachdb/cockroach/blob/f43db402f520dd947d2aa3f742879ba2fe03e048/pkg/server/tenant.go#L1328-L1329

Jira issue: CRDB-52817