Integration tests for CryptoProvider and DigestContext

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
@goffrie introduced a pluggable crypto API (original authored by @tustvold) in this PR:

• Pluggable Crypto / Update reqwest 0.13 #707

Specifically, I am referring to the contents of src/client/crypto.rs and the CryptoProvider and DigestContext traits

The traits are tested, in the sense that there are default providers used, but we don't have any integration tests that show either how to implement these traits externally nor show that you can successfully do so and use the object_store apis

Describe the solution you'd like
Ideally as separate PRs

1. An overview of pluggable crypto in the lib.rs documentation
2. An doc comment example showing how to implement and use a Crypto provider
3. Perhaps an integration test showing how to implement a Cyrpto provider and use it to make actual requests

Describe alternatives you've considered

Additional context

• related to Better testing for different combinations of feature flags, different http clients and crypto libraries #759