sql: add pg_database_size

[BramGruneir]

PGAdmin issues the following query:

SELECT db.datname AS \"Database\", 
       numbackends AS \"Backends\", 
       xact_commit AS \"Xact committed\", 
       xact_rollback AS \"Xact rolled back\", 
       blks_read AS \"Blocks read\", 
       blks_hit AS \"Blocks hit\", 
       tup_returned AS \"Tuples returned\", 
       tup_fetched AS \"Tuples fetched\", 
       tup_inserted AS \"Tuples inserted\", 
       tup_updated AS \"Tuples updated\", 
       tup_deleted AS \"Tuples deleted\", 
       stats_reset AS \"Last statistics reset\", 
       slave.confl_tablespace AS \"Tablespace conflicts\", 
       slave.confl_lock AS \"Lock conflicts\", 
       slave.confl_snapshot AS \"Snapshot conflicts\", 
       slave.confl_bufferpin AS \"Bufferpin conflicts\", 
       slave.confl_deadlock AS \"Deadlock conflicts\", 
       temp_files AS \"Temporary files\", 
       temp_bytes AS \"Size of temporary files\", 
       deadlocks AS \"Deadlocks\", 
       blk_read_time AS \"Block read time\", 
       blk_write_time AS \"Block write time\", 
       pg_database_size(db.datid) AS \"Size\" 
FROM pg_stat_database AS db 
LEFT JOIN pg_stat_database_conflicts AS slave 
  ON db.datid = slave.datid 
WHERE db.datid > NULL::OID 
ORDER BY db.datname

Details here https://www.postgresql.org/docs/current/static/functions-admin.html

Jira issue: CRDB-5014

[jordanlewis]

This should be a straightforward extension of #59604.