Initial PR for adding hooks

[ramasesh]

Adds functionality to have hooks run pre-build and pre-run, as discussed. This is an initial example which works for cloud submission just to get the discussion going.

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[codecov[bot]]

Codecov Report

Merging #93 into master will decrease coverage by 0.73%. The diff coverage is 36.66%.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
- Coverage   55.72%   54.98%   -0.74%     
==========================================
  Files          31       35       +4     
  Lines        3180     3299     +119     
==========================================
+ Hits         1772     1814      +42     
- Misses       1408     1485      +77     

Impacted Files	Coverage Δ
caliban/config/__init__.py	100.00% <ø> (ø)
caliban/hooks/util.py	23.40% <23.40%> (ø)
caliban/docker/build.py	32.74% <30.76%> (+0.03%)	:arrow_up:
caliban/platform/cloud/core.py	23.58% <33.33%> (+0.28%)	:arrow_up:
caliban/hooks/git_commit_hooks.py	34.48% <34.48%> (ø)
caliban/hooks/examples/git_commit_pre_build.py	63.63% <63.63%> (ø)
caliban/hooks/examples/git_commit_pre_run.py	71.42% <71.42%> (ø)
caliban/platform/gke/util.py	99.25% <0.00%> (-0.75%)	:arrow_down:
... and 2 more

---

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[ramasesh]

Note from a conversation with Sam: For the validator hook (which checks that args are well formatted), we can add this as a dry_run hook.

Edit: this idea is bad, we should just encourage a design pattern where the user has, within their src directory, a file akin to train.py which just validates the arguments. Then instead of doing dry_run with a hook, the user just does Caliban run with the file argument_validator.py. Boom, no dry_run hooks.

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