Support of basic interaction with Postgres database

As we are moving toward using Postgres DB as a storage for replication jobs
we need to start creation of the basic components required for interaction with it.
Also it includes funcs and structs that would be helpful in tests.
To test integration with Postgres there is a new CI job 'praefect_sql_test'.
Makefile includes new task 'test-postgres' to run Postgres related tests.

Part of: #2166 (closed)