✨ feat: add pqsql configs for open and idle connections (#1583)

When Postgres is used as the backing database for headscale,
it does not set a limit on maximum open and idle connections
which leads to hundreds of open connections to the Postgres
server.

This commit introduces the configuration variables to set those
values and also sets default while opening a new postgres connection.

config-example.yaml

@@ -153,6 +153,9 @@ database:
   #   name: headscale
   #   user: foo
   #   pass: bar
+  #   max_open_conns: 10
+  #   max_idle_conns: 10
+  #   conn_max_idle_time_secs: 3600
 
   #   # 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.