fix: Allowed multiple update callbacks on an AttrR

[evvaaaa]

• Transports will add an update callback, so if we don't allow for multiple then running them simultaneously won't work.
• The controller level can add their own now too.

[codecov]

Codecov Report

Attention: Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.56%. Comparing base (759fa13) to head (a4c4499).

Files with missing lines	Patch %	Lines
src/fastcs/backend.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
+ Coverage   90.52%   90.56%   +0.03%     
==========================================
  Files          41       41              
  Lines        1920     1928       +8     
==========================================
+ Hits         1738     1746       +8     
  Misses        182      182              

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[jsouter]

Looks good, I'll check if with fastcs-eiger

[evvaaaa]

Looks good, I'll check if with fastcs-eiger

I imagine it wouldn't work because of other changes. We've just found a bug in the last PR where now the controller that comes through in handlers is always the top level one.