Better usability for calculating TTL limit rates

**Is your feature request related to a problem? Please describe.**
When calculating TTL limit rates, the process is pretty nebulous. Ideally, you would be able to specify a per table rate or a per cluster rate and that would be the end of the process. However, that rate can be impacted by factors like the number of nodes in the cluster, the number of tables in the cluster, the delete batch size, and the select batch size. This makes calculating the proper rate for a customer fairly difficult and subject to change when clusters are resized or workloads are added or removed.

**Describe the solution you'd like**
One global cluster limit that is carved up among all TTL workloads would be one potential solution

**Describe alternatives you've considered**
May not be an issue once #98722 is resolved as the general guidance is to allow Admission Control to handle TTL

**Additional context**
Add any other context or screenshots about the feature request here.

Jira issue: CRDB-28134