Add unit of work

[christoph-blessing]

This PR adds the unit of work which governs if and when operations applied to a link are persisted.

Before:

link = gateway.create_link()
result = link.apply(Operations.START_PULL, requested=requested)
gateway.apply(result.updates)
result = link.apply(Operations.PROCESS, requested=requested)
gateway.apply(result.updates)

After:

with uow:
    uow.link.apply(Operations.START_PULL, requested=requested)
    uow.link.apply(Operations.PROCESS, requested=requested)
    uow.commit()

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (705da2d) 98.72% compared to head (508f348) 98.32%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
- Coverage   98.72%   98.32%   -0.41%     
==========================================
  Files          23       24       +1     
  Lines         785      894     +109     
  Branches       95      109      +14     
==========================================
+ Hits          775      879     +104     
- Misses          4        8       +4     
- Partials        6        7       +1     

Files	Coverage Δ
link/infrastructure/link.py	97.36% <100.00%> (+0.14%)	⬆️
link/service/services.py	100.00% <100.00%> (ø)
link/service/uow.py	100.00% <100.00%> (ø)
link/domain/link.py	97.29% <92.00%> (-2.71%)	⬇️
link/domain/state.py	96.45% <94.11%> (-1.95%)	⬇️

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