Add in_ and at helpers to allow for directly requesting a retry

[bunchesofdonald]

Closes #129

[chrisguidry]

The API looks great, and it's super cool we could implement this without changing worker. It does mean that what shows up in the logs is a task failure, which I'm good with right now. We may find that we want these to be treated as "normal" flow-control exceptions and not log them as failures, but this is a great first step.

Another thing I was thinking about: what happens if someone calls ExponentialRetry.in_(...)? Maybe ExponentialRetry shouldn't have a minimum_delay attribute, but re-use the delay attribute from the base Retry?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (0244315) to head (f0d1a6f).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #130   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines         3223      3293   +70     
  Branches       183       186    +3     
=========================================
+ Hits          3223      3293   +70     

Flag	Coverage Δ
python-3.12	100.00% <100.00%> (ø)
python-3.13	100.00% <100.00%> (ø)

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Files with missing lines	Coverage Δ
src/docket/dependencies.py	100.00% <100.00%> (ø)
tests/test_dependencies.py	100.00% <100.00%> (ø)

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[bunchesofdonald]

I believe the docs failure here is because this came in from a fork, rather than being part of this repo directly.