Improve Controller Attribute and sub controller creation with __setattr__

[GDYendell]

Breaking Changes

• register_sub_controller -> add_sub_controller
• sub controllers with snake_case names will now be converted to PascalCase for use in PVs, as attribute names are. If you are currently calling register_sub_controller with a PascalCase name, you can instead add it as an attribute with a snake_case name and the generated PVs will be the same.

Features

• new Controller.add_attribute that can be used instead of setattr(self, ...) - direct access to Controller.attributes may be deprecated in a future release
• sub controllers can be added to a controller by assigning it as an attribute - add_sub_controller can be used directly if adding programmatically or to avoid using a non-snake-case name for an attribute. They will be available via controller.<sub_controller_name> in the interactive shell either way
• when Attributes are added to a controller, the name used is added to its _name field to use in __repr__ so you can tell what they are in log statements and the debugger

[codecov]

Codecov Report

❌ Patch coverage is 96.22642% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.18%. Comparing base (dcd7b5e) to head (46deaa7).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/fastcs/attributes.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #235      +/-   ##
==========================================
+ Coverage   90.15%   90.18%   +0.02%     
==========================================
  Files          44       45       +1     
  Lines        2256     2273      +17     
==========================================
+ Hits         2034     2050      +16     
- Misses        222      223       +1     

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[coretl]

From a skim this looks fine