opt: consider partial indexes for workload index recommendations

**Is your feature request related to a problem? Please describe.**
The index recommendation engine currently does not recommend partial indexes, since it's not clear what predicates should be considered for the index predicate. However, when creating workload index recommendations, we have access to historical queries, so we could see which predicates are used frequently.

**Describe the solution you'd like**
The workload index recommendation engine should identify predicates in the historical queries that were not parameterized and also used regularly. These predicates should be considered for partial index candidates.

**Additional context**
Related slack thread: https://cockroachlabs.slack.com/archives/C01RX2G8LT1/p1742836553047329

Jira issue: CRDB-48828