feat: adapt MQTT bindings to v3

[jonaslagoni]

Description This PR adapts the MQTT bindings to v3. This first round of review is just for the general structure changes.

TODO right before final review:

• [x] Adapt examples, if any that contain v2 structure.
• [x] Figure out which version change this constitutes

TODO after merge:

• [ ] Adapt schema files in spec-schema-repo

Related issue(s) Related to https://github.com/asyncapi/bindings/issues/182

[jonaslagoni]

Thanks folks 👍

[jonaslagoni]

/rtm