Allow Scheduler arguments to be callables for delayed resource determination

[basnijholt]

This is useful when using dependencies and new job's resources should be set dependent on the job.

[codecov]

Codecov Report

Attention: Patch coverage is 90.65421% with 10 lines in your changes missing coverage. Please review.

Project coverage is 74.59%. Comparing base (97b20d8) to head (d682d94).
Report is 10 commits behind head on main.

Files	Patch %	Lines
adaptive_scheduler/_scheduler/slurm.py	90.58%	8 Missing ⚠️
adaptive_scheduler/_server_support/slurm_run.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
- Coverage   74.72%   74.59%   -0.13%     
==========================================
  Files          22       22              
  Lines        2777     2850      +73     
==========================================
+ Hits         2075     2126      +51     
- Misses        702      724      +22     

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