sql/opt: index recommendations can significantly increase planning time

For certain types of complex queries, finding index recommendations can take significantly longer than the time to optimize or even execute the query. This is a problem because when the cluster setting `sql.metrics.statement_details.index_recommendation_collection.enabled` is true (which it is by default), we periodically collect index recommendations as part of execution to populate the DB Console SQL Insights. As a result, every few executions of a query could have high latency due to the time needed to collect index recommendations. This seems to have gotten worse in 23.2+ with https://github.com/cockroachdb/cockroach/pull/103782, which sometimes increases the number of index candidates considered.

We should prevent index recommendations from significantly increasing the latency of production queries. Some ideas:
- Limit the time for index recommendations to some configurable value (maybe 1s by default). Don't collect recommendations if it takes longer than this (make the lack of recommendations due to a timeout visible in the SQL Insights page as well).
- Provide a separate session setting to set the timeout for index recommendations on EXPLAIN.
- Collect index recommendations in a separate goroutine to avoid increasing the latency of production queries.

Jira issue: CRDB-42591

Jira issue: CRDB-45779