sql: Block IMPORT statements for RLS tables

[spilchen]

Data ingestion methods other than INSERT could inadvertently bypass RLS policy enforcement. To align with PostgreSQL behavior, we will prevent IMPORT and COPY operations on tables with row-level security (RLS) enabled.

PostgreSQL returns an error similar to this, which we should replicate:

postgres=> COPY cities FROM '/c.csv';  
ERROR:  COPY FROM not supported with row-level security  
HINT:  Use INSERT statements instead.  

Jira issue: CRDB-45262

[spilchen]

I want to allow the COPY command now since it does go through the query engine. I verified that we still enforce RLS policies. Given this, I'll pivot and add tests to ensure policies are properly enforced.

We still want to block IMPORT though. A PR for that should be coming soon.