make TerraformState classmethods more clear

[marcoceppi]

This PR came about because in a previous update I abused the load classmethod as a way to update TerraformState after the modify_state hook runs. Since it's a classmethod it spawns a new class object instead of loading over it's existing state. This introduces TerraformState.from_file and TerraformState.from_state classmethods which expand on what the load classmethod was doing previously. load now updates the existing invocation and the state parsing code has been moved to a static method.

The two reservations I have and am looking to get explicit feedback on is:

1. Should load be renamed to update in order to better highlight it's purpse?
2. Should from_state be renamed to something like from_str given the data passed to it is likely a string representation of state?

[codecov]

Codecov Report

Merging #20 into master will increase coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   92.27%   92.51%   +0.23%     
==========================================
  Files           8        8              
  Lines         440      454      +14     
==========================================
+ Hits          406      420      +14     
  Misses         34       34              

Impacted Files	Coverage Δ
pytest_terraform/exceptions.py	100.00% <100.00%> (ø)
pytest_terraform/tf.py	96.19% <100.00%> (+0.18%)	⬆️

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[kapilt]

1. load to update, yes. previously it was a constructor returning new instance, now its a mutation that should be reflected as such
2. from_string sounds good, where do we pass in an object just on the hook mutation writer? if we are passing in a object, i would expect a shallow copy rather than a shared copy. ie state_d = dict(state.dict)
3. we should have some typing annotations and/or doc strings inline, the current notion of state (from_state) as Union[TerraformDate, Dict[Any][Any]] is a little implicit to infer else.

[marcoceppi]

1. sounds good, updated
2. nothing actively uses the from_state / from_string except for from_file and some unit tests bits. It's more there to distinguish from_file, used occasionally in the unit tests, and in the event we need it in the future. All the hook mutation bits are here:

   pytest-terraform/pytest_terraform/tf.py

   Lines 345 to 349 in b839062

   	test_api = self.runner.apply()
   	tfstatejson = test_api.save()
   	self.config.hook.pytest_terraform_modify_state(tfstate=tfstatejson)
   	test_api.load(tfstatejson)
   	test_api.save(module_dir.join("tf_resources.json"))

   where we save the state to a new TerraformStateJson object, use that shared object because of how pytest hooks work, then update the TerraformState with any changes from the hook execution
3. Yeah, this is a good point. I can add type annotations and doc strings

[marcoceppi]

Update TerraformState and TerraformStateJson with annotations and docstrings

[kapilt]

thanks, lgtm