Define DID document metadata property for providing proofs of correctness

[wip-abramson]

Based on the discussion in #102, it would be useful for DID resolution to define a standard property in the DID document metadata for different DID methods to use to include proofs of correctness for the state of the DID document returned from a resolution request.

While the property name would be standard, the contents of that property and how it should be interpreted and verified would be left to the individual DID methods to define.

This property would be optional, but if DID methods have a mechanism to produce these proofs as part of their resolution process they SHOULD use this property.

We would just need to define it in this section https://w3c.github.io/did-resolution/#did-document-metadata

We also need some text around how this property could be used/extended by specific DID methods