Make assumed_state for RTS configurable via option flow

[iMicknl]

Current behaviour is that assumed_state is True for all rts devices, since RTS doesn't have a feedback function.

However, when you are only controlling your RTS devices via Home Assistant, some people don't want to have this optimistic mode.

[iMicknl]

A setting is not needed, can be tweaked already via core. https://www.home-assistant.io/docs/configuration/customizing-devices/#assumed_state

[vlebourl]

Tested and works as expected with customize.yaml.

[iMicknl]

@vlebourl great! We need to add it to the README at some point, same for the device class that could be changes via customize.yaml.

[vlebourl]

We can add it to the README, although it's a standard feature of HA, every entity can be customized. Not sure whether we should be that thorough in our README.