Allow to specify full file name in value reference '!' (#14955)

* Extract parsing of value reference '!' in settings into a function

* Allow to specify full file name in value reference '!'

This `network_key: '!secret network_key'` is really confusing because
it looks like a YAML tag named 'secret' and a value "network_key".
When we write `network_key: '!secret.yaml network_key'` instead, it's
a little clearer what's going on. Also, it's more consistent with file
references in 'devices' and 'groups'.

test/settings.test.js

@@ -199,7 +199,7 @@ describe('Settings', () => {
             mqtt: {
                 server: '!secret server',
                 user: '!secret username',
-                password: '!secret password',
+                password: '!secret.yaml password',
             },
             advanced: {
                 network_key: '!secret network_key',