Implement a publicMethodFor() helper function.

[dmitrizagidulin]

This PR adds a convenience function that can be used in conjunction with a .get() to start with a did:key DID, and end up with a key for a specific purpose.

For example,

// Start with the DID
const didDocument = await didKeyDriver.get({did});
// This lets you use `publicMethodFor()` to get a key for a specific purpose
const keyAgreementData = didKeyDriver.publicMethodFor({
  didDocument, purpose: 'keyAgreement'
});
const assertionMethodData = didKeyDriver.publicMethodFor({
  didDocument, purpose: 'assertionMethod'
});
// If you're using a `crypto-ld` driver harness, you can create key instances
// which allow you to get access to a `verify()` function.
const assertionMethodPublicKey = await cryptoLd.from(assertionMethodData);
const {verify} = assertionMethodPublicKey.verifier();

This addition matches the convenience methodFor function that gets returned with generate(). The main difference is -- since it's returned by generate(), methodFor has access to the private key material (and so, can be used for decryption, signing, etc).
Whereas publicMethodFor only has access to the public key material (extracted from the DID Document), and is useful by consumers that need those keys for verification and encryption (to a known DID recipient).

[dlongley]

I have no idea how useful this is to devs (not having contact with this code in a while). I'm presuming people are needing this ... in which case I approve.

[mattcollier]

Why not return a NotFoundError? Otherwise the consumer of this API has the null check the response right?

[dmitrizagidulin]

Throwing an error seemed too drastic of a response (since not having a key in a did document for a given purpose is a normal condition). I could add that, if you feel strongly about it.

[mattcollier]

I do think an error would be better. I'm having difficulty imagining how returning undefined does not lead to an error anyhow. Likely the typical cannot access foo of undefined error.

Did you have some specific use pattern in mind?

[dmitrizagidulin]

@mattcollier Np; changed to throw error when not found. Thanks!

[mattcollier]

Question about return value of publicMethodFor

[mattcollier]

Approving, just a few more nits.

[mattcollier]

Suggested change

	expect(error).to.be.exist;
	expect(error).to.exist;

[dmitrizagidulin]

Whoops.

[mattcollier]

can avoid regex with .to.contain()

[mattcollier]

This needs updating now.

[dmitrizagidulin]

@mattcollier Thanks for the catch; all updated.