implement ValidatedTopic for mqttv5 (needs feedback)

[CramBL]

Resolves https://github.com/bytebeamio/rumqtt/issues/979

Only implemented for v5, I'll implement it for v3 as well if we agree on the design and implementation, and add changelog entry, tests, refactor it out to a separate file, add an example, etc.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• [ ] Formatted with cargo fmt
• [ ] Make an entry to CHANGELOG.md if it's relevant to the users of the library. If it's not relevant mention why.