opt: Add optimizer support for UPSERT and INSERT..ON CONFLICT statements

[jseldess]

PR: cockroachdb/cockroach#33339

From release notes:

The new experimental_optimizer_updates cluster setting controls whether UPDATE and UPSERT statements are planned by the cost-based optimizer rather than the heuristic planner. Also note that when the setting is set, check constraints are not checked for rows skipped by the INSERT ... DO NOTHING clause. [#33339][#33339] {% comment %}doc{% endcomment %}

[rmloveland]

This was done as part of the work that closed #3998 and is already live in the CBO docs.