[pg] Add current database to pg_database

[rekby]

1. Run Local YDB https://github.com/ydb-platform/ydb/wiki/Local-run-postgres-tests

echo "SELECT datname FROM pg_database" | psql postgres://root:1234@localhost:5432/local

Result:

  datname
-----------
 template1
 template0
 postgres
(3 rows)

Expected result:

  datname
-----------
 postgres
 local
 template1
 template0
(4 rows)

Notes:

1. Get current database from connection info (pg_wire)
2. Add received database to pg_database table content:

• comp_factory.cpp
• increase row count comp_factory.cpp
• Add oid to syscache.cpp

Similar to: #3124