detect/mqtt: move keywords to rust

detect/mqtt: move keywords to rust

Ticket: 4863

On the way, convert some keywords to use the first-class integer
support.
And helpers for pure rust the support for multi-buffer.

Move the C unit tests about keyword mqtt.protocol_version
to unit tests for generic integer parsing, and test version 5
instead of testing twice version 3.

Also iterate all tx's messages for reason code as is done for other
keywords.

And allow detection on empty topics.