Add support for retrieving central heating flow temperature (OpenTherm)

[strix-technica]

Since October 2022, API endpoints have existed to set the maximum flow temperature and get the last recorded flow temperature on OpenTherm installations (at the least).

This PR adds support for reading current flow temperature (API and CLI). I haven't added a setter, but it looks like it would not be difficult.

A test is included, and I also updated test_devices to cope when not all the devices checked are available in this instance.

[strix-technica]

Ah. The API endpoint in question uses the bridge's "Auth Code" for authentication, rather than the bearer token used by the rest of the API, because it is intended for use by installers etc who don't have access to the user's account. For the unit test, therefore, I added environment variable TADO_BRIDGE_AUTHKEY so this API call can be tested.

I've just noticed that either the test workflow is going to need modification to import this from github secrets, or the unit test will need to be modified to skip that test if this env var isn't defined. Let me know what, if anything, you'd like me to do further on that.

[germainlefebvre4]

Thank you very much for paying attention to this lib :) I appreciate contributions from the open source community <3

I will take a look at this in the next few days

[strix-technica]

Acknowledged, sorry about the delay, will get to it ASAP.