add pgsql backend

[commonism]

the changes require the postgres user to be able to connec to the database via localhost using password, peer authentication would require using become.
This may require changes to pg_hba.conf and setting a postgres user password:

- name: PostgreSQL | pg_hba.conf
  community.postgresql.postgresql_pg_hba:
    dest: /etc/postgresql/13/main/pg_hba.conf
    contype: host
    users: postgres
    source: "{{item}}"
    databases: ALL
    method: md5
    create: true  
  loop: ["::1/128","127.0.0.1/32"]

- name: PostgreSQL | postgres password
  become: yes
  become_user: postgres
  community.postgresql.postgresql_user:
    user: postgres
    password: "{{ pgsql_root_password }}"
    login_unix_socket: /var/run/postgresql

[pieterlexis]

Sweet! If you could add some tests, I'll happily merge it :)

[lessfoobar]

when will that be merged?

[Exchizz]

I've created a new PR that includes these changes and some tests.

#211