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_char_max_length builtin function is now supported, which improves compatibility with PostgreSQL.

Jira Issue: DOC-1723

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