[FEATURE] cust mqtt v5 or v3 only.

**Is your feature request related to a problem? Please describe.**
compile time cust v3 or v5 only to reduce code size

**Describe the solution you'd like**
A feature toggle.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.