fix: protocol and refactor for custom potential

[janfb]

fixes #1223 by introducing a protocol for custom potentials. This will expose the arguments that we expect for the custom potential and eliminates the need to explicitly check for arguments.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 29.63%. Comparing base (18f92b1) to head (8408298).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
sbi/inference/potentials/base_potential.py	83.33%	1 Missing ⚠️

❌ Your project status has failed because the head coverage (29.63%) is below the target coverage (70.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1409       +/-   ##
===========================================
- Coverage   89.31%   29.63%   -59.68%     
===========================================
  Files         119      119               
  Lines        8779     8914      +135     
===========================================
- Hits         7841     2642     -5199     
- Misses        938     6272     +5334     

Flag	Coverage Δ
unittests	29.63% <87.50%> (-59.68%)	⬇️

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Files with missing lines	Coverage Δ
sbi/inference/posteriors/base_posterior.py	61.72% <100.00%> (-25.93%)	⬇️
sbi/inference/potentials/base_potential.py	84.78% <83.33%> (-6.89%)	⬇️

... and 96 files with indirect coverage changes

[manuelgloeckler]

Nice, thanks for adding. Much cleaner then the old CallableWrapper.

Some minor docstring changes can be made, bot happy to merge it as is.