mqtt: add mTLS authentication using certificates to MQTT

[tomjschwanke]

This implements mTLS for MQTT in the backend. A custom CA-Cert can be used for validating the identity of the broker and a Client-Cert + Client-Key can be used for authentication.

In the yaml these values can be set using cacert, clientcert and clientkey. Example:

mqtt:
  cacert: |
    -----BEGIN CERTIFICATE-----
    [CERTIFICATE DATA]
    [CERTIFICATE DATA]
    -----END CERTIFICATE-----
  clientkey: |
....

Values saved in the database are also used correctly (added caCert, clientCert, clientKey as strings to the MQTT JSON object)

~~Migrating values from the YAML to the DB is not working yet!~~ ~~failed parsing config file: 'Mqtt' has invalid keys: cacert, clientcert, clientkey~~

Configuration through the UI is not supported yet, but adding the caCert, clientCert and clientKey string values to the MQTT JSON payload of the REST request works.

Not sure if a clientKey is always needed for a clientCert, but currently it only uses the cert if a key is also specified