sql: support listing grants for role 'public'

[fabiog1901]

Currently, there is no simple way to list the GRANTS given to role public.

The SQL statement below helps

select * 
from [show grants] 
where grantee = 'public' 
    and (schema_name is null or schema_name = 'public');

but ideally I'd like to run the standard show grants stmt, as in

show grants for public;

Slack

Jira issue: CRDB-24427

Epic CRDB-17785