sql: implement information_schema.{_pg_truetypid, _pg_truetypmod, _pg_char_max_length}

[cockroach-teamcity]

Exalate commented:

cockroachdb/cockroach#69913 --- Release note (sql change): The information_schema._pg_truetypid and information_schema._pg_truetypmod builtin functions are now supported, which improve compatibility with PostgreSQL.

Jira Issue: DOC-1724