Allow WorkerSettings inheritance

[ykvch]

Read Worker class attibutes (settings) from partent classes.
This way one may write base worker to settings as:

class BaseSettings:
    on_startup = startup
    on_shutdown = shutdown

class WorkerSettings(BaseSettings):
    functions = [download_content]

and allow multiple workers to reuse base settings.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
arq/worker.py	33.33%	1 Missing and 1 partial ⚠️
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@@            Coverage Diff             @@
##             main     #511      +/-   ##
==========================================
- Coverage   96.27%   95.82%   -0.45%     
==========================================
  Files          11       11              
  Lines        1074     1079       +5     
  Branches      209      192      -17     
==========================================
  Hits         1034     1034              
- Misses         19       22       +3     
- Partials       21       23       +2     

Files with missing lines	Coverage Δ
arq/worker.py	96.77% <33.33%> (-0.40%)	⬇️

... and 1 file with indirect coverage changes

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