Maximum packet size in MQTTv5

[swanandx]

This PR adds support for max packet size as per defined in MQTTv5 standards.

Type of change

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

Checklist:

• [x] 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.