Add handling subclassed evidence support

[alfinkel]

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What

• Add support for check decorators and context manager to return subclassed framework evidence objects.
• Replace os.path with pathlib.PurePath in the evidences module
• Add a content_as_json property to evidence base class

Why

• The act of casting of subclassed raw or external evidence should be handled by the framework instead of having to be handled by individual check logic.
• Using PurePath or Path from pathlib whenever possible over os.path is a cleaner more generic solution.
• Adding content_as_json is also helpful and a common enough need that it should be contained within the framework for evidences with JSON content

How

• Add FromEvidence named tuple
• Add passing FromEvidence to with_*_evidences decorators
• Add passing FromEvidence to evidences context manager
• Replace os.path usage with pathlib.PurePath
• Updated docs

Test

• Present behavior of with_*_evidences decorators and evidences context manager is preserved
• Passing a combination of FromEvidence named tuples and string paths to decorators and context manager yields the expected results for evidences returned. Evidences specified as FromEvidence are returned to the test_ function/method as the evidence specified as the ev_class otherwise the evidence is returned as either raw or external as appropriate.

Context

N/A

[alfinkel]

@cletomartin as discussed over Slack, at your suggestion, I've changed the FromEvidence named tuple functionality to be part of a class method (lazy_load) now found on all Evidence classes. This removes the burden of having to import FromEvidence every time you'd want to use it. I did however change things a bit in that FromEvidence is now LazyLoader and it is at the evidence module level rather than inside the base evidence class as we had originally discussed. The reason for that is the with_*_evidences decorators and evidences context manager need to reference it in order to ensure proper processing.

[drsm79]

Nice

[drsm79]

I think this could use an explanatory comment - if I'm reading it right, it'll make an instance of the class defined in the lazy loader if that class is a subclass of the type_str, otherwise will make the evidence be of the type in type_str.

How about:

Suggested change

	if issubclass(from_ev.ev_class, get_evidence_class(type_str)):
	ev_class = get_evidence_class(type_str)
	if isinstance(from_ev, LazyLoader):
	path = from_ev.path
	if issubclass(from_ev.ev_class, get_evidence_class(type_str)):
	ev_class = from_ev.ev_class

and save an if.

[alfinkel]

Done in a96efa7

[drsm79]

I wonder if pathlib could be used here to avoid the if: & the fstring.

[alfinkel]

Done in a96efa7 and 70751fd