pmm-admin changes for new pg_stat_monitor PostgreSQL extension

As part of implementing pg_stat_monitor we should implement pmm-admin part.

What should be done:

• PMM-Admin
  • Add new inventory command `pmm-admin inventory add agent qan-postgresql-pgstatmonitor-agent`
  • Add new query source for pmm-admin add postrgesql command
  • Show pg_stat_monitor in pmm-admin list agents command.

How to test:

• First case
  • Run `pmm-admin add postgresql` with pg_stat_monitor query source
  • Run `pmm-admin inventory list agents` and see newly created agent
• Second Case
  • Run `pmm-admin inventory add agent qan-postgresql-pgstatmonitor-agent` with pg_stat_monitor query source
  • Run `pmm-admin inventory list agents` and see newly created agent