Signature support

[trashhalo]

It would be nice if the crypto piece of charm could expand to signing data with my identity and verifying signed data.

Im specifically thinking about use cases like:

https://github.com/dominictarr/scuttlebutt
https://www.w3.org/TR/activitypub/
https://saltpack.org/
https://book.keybase.io/docs/cli#signing

where data is replicated and verified using signatures.

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Requirements:

1. As a charm user, I would like to be able to generate a signature using my identity from an arbitrary set of bytes.
2. As a charm user, I would like to extract an identity and validate a set of bytes using a signature.