Job retention and cleanup

[adam-fowler]

This is an extension of #29

• It combines cleanup and processDataRetentionPolicy and moves the decision on age to the parameters of the cleanup function
• It also starts the work of setting up a scheduled job for running cleanup, but this work needs Resurrect separation of job name and parameters swift-jobs#101 to be complete

[codecov]

Codecov Report

Attention: Patch coverage is 93.38843% with 8 lines in your changes missing coverage. Please review.

Project coverage is 89.71%. Comparing base (857e17b) to head (d2b80d4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...urces/JobsPostgres/PostgresJobsQueue+cleanup.swift	91.02%	7 Missing ⚠️
Sources/JobsPostgres/PostgresJobsQueue.swift	97.22%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
+ Coverage   88.59%   89.71%   +1.11%     
==========================================
  Files           5        7       +2     
  Lines         456      525      +69     
==========================================
+ Hits          404      471      +67     
- Misses         52       54       +2     

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