Garbage collection for Update objects

[EronWright]

Proposed changes

Implements a retention policy for completed Update objects, based on a simple TTL with the default value of 24h. Update objects that are in a completed state and are older than the TTL are summarily deleted.

New API: Update.spec.ttlAfterCompleted

A new field to enable garbage collection of this update, once completed. Optional; no default value.

Update Controller

Updated to implement the cleanup logic. When a TTL is set on a completed update, the controller looks to the last transition time of the Completed status condition. If there's time remaining, the controller requeues the object for later, otherwise it deletes the update object.

Stack Controller

Updated to assign a 24-hour TTL on the update objects that it creates. A future enhancement would be to provide an updateTemplate field to allow for further control. A user may edit an existing Update to change the TTL.

Update Controller Tests

A new unit test was added to exercise the retention logic.

Related issues (optional)

Closes #733

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 3 lines in your changes missing coverage. Please review.

Project coverage is 51.61%. Comparing base (1795411) to head (3551638).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
...ator/internal/controller/auto/update_controller.go	78.57%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #810      +/-   ##
==========================================
+ Coverage   51.16%   51.61%   +0.44%     
==========================================
  Files          31       31              
  Lines        4325     4340      +15     
==========================================
+ Hits         2213     2240      +27     
+ Misses       1923     1909      -14     
- Partials      189      191       +2     

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[thomas11]

LGTM as someone who isn't familiar with PKO.

[thomas11]

[nit] cleanup? Also line 53

[EronWright]

I deliberately kept this here because it is useful scaffolding for more tests. This controller simply doesn't have any tests yet that would use it. Keep/drop?