Replace Handler with AttributeIO / AttributeIORef

[jsouter]

Closes #211

all the tests are currently broken, have not fully removed handlers yet.
So far mostly copying https://github.com/GDYendell/fastcs2 with a few minor changes

• for the time being i've removed one of the generic args from AttributeIOs, I intend to recover them at some point
• using get_args to inspect the generic args for AttributeIO to prevent having to call super().__init__(ref)
• using TypeVar from typing_extensions which allows us to have a default value (this is nice for Attrs but might be problematic for AttributeIOs in the current implementation, will think about this a bit more)

very wip

[codecov]

Codecov Report

❌ Patch coverage is 94.04762% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.27%. Comparing base (9c2042a) to head (3948522).

Files with missing lines	Patch %	Lines
src/fastcs/attribute_io.py	85.00%	3 Missing ⚠️
src/fastcs/attributes.py	96.00%	1 Missing ⚠️
src/fastcs/backend.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #218      +/-   ##
==========================================
- Coverage   92.34%   92.27%   -0.08%     
==========================================
  Files          40       42       +2     
  Lines        2077     2122      +45     
==========================================
+ Hits         1918     1958      +40     
- Misses        159      164       +5     

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[jsouter]

Have confirmed that the tickit demo/IOC works. Got to clean up comments and squash a bit, probably want a few extra tests too.