Implement PostgreSQL monitor type

[aldy505]

Uptime Kuma (the upstream of this package) provides more monitor types than just HTTP call or ICMP ping like Roselite currently supports.

As a part of extending the monitor type that's also supported by Roselite, it would be very nice if we can provide more than what we currently supports.

For this issue, we're tracking PostgreSQL monitor supports. According to the image below the thing that we can configure is:

• Connection string (in the form of postgres://username:password@hostname:port/database_name?config=value&more_config=more_value. An example would be: postgres://root:[email protected]:5432/foobar?sslmode=disable).
• Custom query. We should make this as optional, and provides SELECT 1 as a default query.