sql: Include policy information in explain output

[spilchen]

When you explain a query, we want to surface information about any row-level security (RLS) policies that will apply. It is expected that users will use this output as a way of ensuring the proper RLS policies are in force.

To avoid including too much RLS information, we will only expose it if the VERBOSE option is used.

The output will look something like this:

> explain (verbose) select * from t1;
                                    info
----------------------------------------------------------------------------
...
  row-level security: enabled
  row-level security policies: t1 => permissive [p1, p2]; restrictive [p3]
...

RLS information should be visible for all SELECT, UPDATE, DELETE, INSERT DML.

Jira issue: CRDB-46366