Flag duplicate indexes in tables in Index Recommendations and Schema Insights

**Is your feature request related to a problem? Please describe.**
CRDB currently gives index recommendations for various scenarios including adding missing indexes or removing unused indexes. I think it would be helpful (and probably pretty simple) to flag these:

**Describe the solution you'd like**
If a table exists like this:
```
CREATE TABLE x (
id int primary key,
type string,
INDEX ix1 ( type ASC ),
INDEX ix2 ( type ASC )
);
```
(i.e., where there are two or more indexes with the exact same index definition) that we could flag these and recommend that these duplicates be dropped.

**Describe alternatives you've considered**
I suppose one could write a query to hit crdb_internal tables and find these and run it every once in a while.

Jira issue: CRDB-33328

The issue names Index Recommendations and Schema Insights but no files or tests. Start by locating those components and their existing missing- and unused-index recommendation paths. Done means tables with exact duplicate index definitions are flagged and duplicate indexes are recommended for removal.

Written by the indexing model from the issue text.