Add embeded authentications

[paulobmarcos]

Authentication

An Authentication method can be embedded or referenced.

The DID spec, at the time of this issue, does not fully cover what and how an authentication method should be verified.

An example of a verification method is a public key.

This raised some questions during the first implementation of the module:

• Can they only reference/embed public keys? If not, what more can they reference/embed?
• Are this public keys from the same document?
• If a public key is referenced from another document, should we manage a way to resolve it?
• What are the use cases for embedded public keys? Are they available to avoid resolving a public key?

After some thought, me and @satazor decided that, during the first implementation, we would only support referenced authentication methods based on public keys from the same document.

We hope the DID spec to mature in following months and better cover the questions above.