Adding support for setting PriorityLevel and PriorityLevelLock in GW_COMMAND_SEND_REQ

[kevinior]

I'm thinking about adding support for setting the PriorityLevel and PriorityLevelLock in GW_COMMAND_SEND_REQ frames. I'd like to be able to have Home Assistant act as a fake rain sensor for my awnings, by forcing them to go up when rain is forecast soon.

In the pyvlx code I can see that FrameCommandSendRequest already has a field for priority, but it's always set to a default value and the priority locking fields are always sent as zeroes.

Would it be OK if I create a pull request implementing this?