Restructure database config

[kradalby]

This PR restructures the database part of the configuration to follow the indent style of the rest of the config. It gives a better indication for which setting belongs to sqlite and postgres.

It is intended for making adding specific features like #1583 less confusing.

The old keys are aliased to the new settings keys so this is not breaking, but marked as deprecated in the changelog so we can remove them later.

New config example:

database:
  type: sqlite3

  # SQLite config
  sqlite:
    path: /var/lib/headscale/db.sqlite

  # # Postgres config
  # postgres:
  #   # If using a Unix socket to connect to Postgres, set the socket path in the 'host' field and leave 'port' blank.
  #   type: postgres
  #   host: localhost
  #   port: 5432
  #   name: headscale
  #   user: foo
  #   pass: bar

  #   # If other 'sslmode' is required instead of 'require(true)' and 'disabled(false)', set the 'sslmode' you need
  #   # in the 'db_ssl' field. Refers to https://www.postgresql.org/docs/current/libpq-ssl.html Table 34.1.
  #   ssl: false

[pallabpain]

Looks good. A few minor comments around refactoring the code for the database config.