Allow User Properties for MQTT V5 Protocol to be used as placeholders

[estebanz01]

Hola! 👋

Let me know if this is not the right place to post this feature request. I'm posting this request based on this discussion: emqx/emqx#11528

Basically, with the introduction of User Properties in the V5 version of the MQTT protocol, I think it expands the possibility to enhance authentication processes by allowing the specified properties to be used as placeholders in different authentication/authorization services.

for my particular use-case, I want do to something like this:

• A client sets a User-Property of (ApiKey, ABC123) in the CONNECT packet along with a username and a password.
• EMQx sees this and sends as a POST request a username, password and ApiKey: ABC123 to an external HTTP service.
• The HTTP Service receives a JSON object like this: {"username": ..., "password": ..., "apiKey": "ABC123" }.

Or expose them via headers too, so the HTTP Endpoint receives a custom header like X-API-KEY: ABC123.

Also, allowing user properties to be used as placeholders, will allow more granular control over the PUBLISH packet as well with ACLs.

Thanks!