RS485 serial support / tx enable pin control

**Describe the problem you have/What new integration you would like**

I would like to have a RS485 serial component available which extends the serial component with a half-duplex tx pin enable option. In order to use a shared bus, rs485 requires setting a tx enable pin when transmitting.

**Please describe your use case for this integration and alternatives you've tried:**

My particular use case is integration with Carrier thermostats which use a custom protocol over RS485(toy C implementation here https://github.com/nebulous/infinitude/tree/master/contrib/cardump), but this would be applicable to any half-duplex serial system. While modbus doesn't particularly interest me, an rs485 component would also form a standard base for those who wish to implement modbus components.

**Additional context**

Most [cheap rs485 interfaces based on the MAX485](https://amzn.to/2LVSmGe) break out DE and RE pins, which typically are shorted together into one "I'm currently transmitting" pin. Bonus points to an implementation which accounts for disablement of rx interrupts while transmitting. That is, independent monitoring/driving of DE/RE.

Here's a reference patch to a similar implementation for mongooseOS https://github.com/cesanta/mongoose-os/commit/a71bfbade5964601b004bc3f1de8d75616e46137