Add `Algorithm.compute_hash_digest` and use it to implement at_hash validation example (#775)

* Add compute_hash_digest to Algorithm objects

`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 classes
with a class decorator) but this is not clearly a worthwhile
optimization. Such perf tuning is intentionally omitted for now.

* Add doc example of OIDC login flow

The goal of this doc example is to demonstrate usage of
`get_algorithm_by_name` and `compute_hash_digest` for the purpose of
`at_hash` validation. It is not meant to be a "guaranteed correct" and
spec-compliant example.

closes #314

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