Add SimpleHandler to make minimal Attributes functional

[GDYendell]

This allows creating Attributes without a Handler for parameters that are only used for internal controller logic and not sent directly to a device.

Fixes #101

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.76%. Comparing base (2153173) to head (d789693).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/fastcs/attributes.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #105   +/-   ##
=======================================
  Coverage   87.75%   87.76%           
=======================================
  Files          28       28           
  Lines        1184     1193    +9     
=======================================
+ Hits         1039     1047    +8     
- Misses        145      146    +1     

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[d-perl]

This is great, exactly what I needed, thank you!