Create ControllerAPI class to decouple transport from Controller

[GDYendell]

This changes a few key parts of the internal logic to make the static typing more robust and to improve the driver API.

It is now possible to dynamically create Methods as well as Attributes. This is shown in the new test here where a Command is added to a Controller during initialise.

The new Command, Put and Scan methods enable decoupling the Controller from the transport layer via a new ControllerAPI class, which replaces and extends the old SingleMapping class. The differences being:

• ControllerAPI doesn't need a reference to the Controller - it is entirely self contained because the methods are dynamically bound to the controller instance and can be called outside the context of the class.
• ControllerAPI maintains the tree structure from the Controllers where SingleMappingflattened it

Summary of changes:

• Controller.get_controller_mappings -> ControllerAPI.walk_api
• SingleMapping.{attributes,command_methods,put_methods,scan_methods} -> ControllerAPI.{attributes,command_methods,put_methods,scan_methods}
• SingleMapping.Controller.path -> ControllerAPI.path
• {Command,Put,Scan}.fn is now callable directly. Don't do MethodType(method, controller) or getattr(controller, method.__name__)().
• for Controller in SingleMapping.Controller.get_sub_controllers() -> for ControllerAPI in ControllerAPI.sub_apis()

Fixes #74
Fixes #60
Fixes #93
Fixes #62

[GDYendell]

• Update Command and BoundCommand to UnboundCommand and Command.

[codecov]

Codecov Report

Attention: Patch coverage is 96.70782% with 8 lines in your changes missing coverage. Please review.

Project coverage is 89.00%. Comparing base (88f80a2) to head (973d2bf).

Files with missing lines	Patch %	Lines
src/fastcs/backend.py	91.30%	2 Missing ⚠️
src/fastcs/transport/epics/adapter.py	50.00%	2 Missing ⚠️
src/fastcs/controller_api.py	97.29%	1 Missing ⚠️
src/fastcs/transport/epics/docs.py	66.66%	1 Missing ⚠️
src/fastcs/transport/epics/gui.py	88.88%	1 Missing ⚠️
src/fastcs/wrappers.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
+ Coverage   87.84%   89.00%   +1.16%     
==========================================
  Files          33       34       +1     
  Lines        1456     1519      +63     
==========================================
+ Hits         1279     1352      +73     
+ Misses        177      167      -10     

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