Add option to limit index creation

[jeffreyscarpenter]

When deploying in an managed environment such as Astra, operators need to be able to limit the number of indexes that JSON API creates.

From a practical perspective, the number of indexes is the bigger limiting factor than the number of collections, since the current implementation creates 8 or 9 SAI indexes per collection (Cassandra table), and could potentially create additional indexes once we add text search. Note: Astra currently enforces a lower limit on the number of indexes per keyspace (100) than the number of tables (200).

We need to be able to specify a limit on the number of indexes created. The current number of indexes should be checked prior to creating a table so that the limit is not exceeded while creating a collection. To account for the possibility of simultaneous collection creation, we may also need to implement recovery behavior such that failure to create an index for a new collection results in removal of the table and any indexes already created.

[jeffreyscarpenter]

One relatively simple way to do the check might be to query the system_schema.indexes table (which should give a different value per tenant when running in multi-tenant mode)

SELECT count(*) FROM system_schema.indexes;