feat: Add option to connect as mock devices

[rtuck99]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.75%. Comparing base (6115b04) to head (35f50f3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1245      +/-   ##
==========================================
+ Coverage   94.67%   94.75%   +0.07%     
==========================================
  Files          41       41              
  Lines        2611     2612       +1     
==========================================
+ Hits         2472     2475       +3     
+ Misses        139      137       -2     

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[DiamondJoseph]

Do you have a use case that requires this? To my mind this should be tracked in the dodal module, as the device_factory allows setting whether to connect sim or not, rather than having multiple potentially overlapping places to define this.

[tpoliaw]

There's currently no way (that I'm aware of?) of marking all device factories as mocks in one change so I can see the motivation for being able to mock an entire beamline in one flag.

This is all about to change though if the device manager change works out so it might be easier to change there? Other than the weirdness it introduces around having mock plans, I'm not against the idea of it being here as well though.

[tpoliaw]

Having a mock field on a plan module is odd. Can we split the Source type into a union where each sub type only has relevant fields?