Add Algorithm.compute_hash_digest and use it to implement at_hash validation example

[sirosen]

Algorithm.compute_hash_digest is defined as a method which inspects the object to see that it has the requisite attributes, hash_alg.

If hash_alg is not set, then the method raises a NotImplementedError. This applies to classes like NoneAlgorithm.

If hash_alg is set, then it is checked for

has_crypto  # is cryptography available?
and isinstance(hash_alg, type)
and issubclass(hash_alg, hashes.HashAlgorithm)

to see which API for computing a digest is appropriate -- hashlib vs cryptography.hazmat.primitives.hashes.

These checks could be avoided at runtime if it were necessary to optimize further (e.g. attach compute_hash_digest methods to classeswith a class decorator) but this is not clearly a worthwhile optimization. Such perf tuning is intentionally omitted for now.

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Add doc example of OIDC login flow to demonstrate get_algorithm_by_name and compute_hash_digest.
The end goal is an example with at_hash validation. It is not meant to be a "guaranteed correct" and spec-compliant example, so the doc notes simply that additional reading about OIDC is recommended.

closes #314

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If the example usage is too bulky and complicated, I'm happy to pull that and only focus on the new method on Algorithm, or to pull that out into a separate PR. It seemed more efficient to just put both changes together.

[jpadilla]

Sorry for the delay, I think we can just get this in, surface area seems small enough. The docs and examples are a great addition here, thanks.

Just have to update the changelog on this one.