feat: add encryption key support for ethr-did-documents

[mirceanis]

What's here

This adds explicit support for listing X25519 encryption keys in ethr-did documents
This extends the supported attribute values to include X25519 key algorithm and enc key purpose.

(fixes #52)

Example Base64 encoded X25519 Encryption Key

A DIDAttributeChanged event for the identity 0xf3beac30c498d9e26865f34fcaa57dbb935b0d74 with the name
did/pub/X25519/enc/base64 and the value of
0x302a300506032b656e032100118557777ffb078774371a52b00fed75561dcf975e61c47553e664a617661052
generates a PublicKey entry like this:

{
  id: "did:ethr:0xf3beac30c498d9e26865f34fcaa57dbb935b0d74#delegate-1",
  type: "X25519KeyAgreementKey2019",
  owner: "did:ethr:0xf3beac30c498d9e26865f34fcaa57dbb935b0d74",
  publicKeyBase64: "MCowBQYDK2VuAyEAEYVXd3/7B4d0NxpSsA/tdVYdz5deYcR1U+ZkphdmEFI="
}

Note

Some key algorithms would not fit certain purposes (like encryption with secp256k1 or signing with X25519).
The DID resolver makes no assumptions here and does not enforce a match, nor does the registry contract where these attribute changes need to be sent.
It is up to the owners of the DIDs which attributes they list.

[simonas-notcat]

Looks good

[awoie]

LGTM

[uport-automation-bot]

🎉 This PR is included in version 2.3.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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