Operation code for mesh model messages.
Opcodes are encoded as:
1 byte: 0x00-0x7F (standard SIG models, no company ID)
2 bytes: 0x8000-0xBFFF (standard SIG models)
3 bytes: 0xC00000-0xFFFFFF (vendor-specific models)
Number of bytes needed to encode this opcode (1, 2, or 3).
Encode this opcode to bytes.