Add mock config server fixture

[jacob720]

Needed for DiamondLightSource/mx-bluesky#1228 and sub-issues.

Instructions to reviewer on how to test:

1. Confirm real config server not used in tests.

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards
• If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
• Have the connection tests for the relevant beamline(s) been run via dodal connect ${BEAMLINE}

[oliwenmandiamond]

Instead of being in conftest.py, would this be better in dodal.testing.fixtures? That way, external repos like sm-bluesky and mx-bluesky can reuse it for tests as well via pytest_plugins.

Or do you see it only being needed here?

[oliwenmandiamond]

Better yet, why not include this fixture inside daq-config-server/testing/fixtures and then dodal and whatever else can reuse the fixture via pytest_plugin

[jacob720]

Yeah that's a good idea, we have a pretty much identical fixture in mx-bluesky so will definitely be good to make it reusable