Add an option to connect to devices with mock=True

[d-perl]

When doing end-to-end testing, e.g. calling plans from my new GUI, I'd like to be able to run an instance of BlueAPI with mock devices to run plans against.

I've looked through the code a bit and can't find anything like it, but if it already exists then this issue is to add it to the docs.

[callumforrester]

This is a good idea and important. @fajinyuan has expressed a desire for a similar feature before.

Possible Solution

Add a mock flag to the env config, if it is True pass mock=True to make_all_devices in BlueskyContext

env:
    mock: true

Acceptance Criteria

• There is a mock flag in config.env
• It defaults to False
• If it is set to True, all (ophyd-async) devices are connected in mock mode