Update PortableDid structure

[frankhinek]

Context here: decentralized-identity/web5-spec#112 (comment)

PortableDid should be changed to:

{
    uri: "",
    privateKeys: [{...jwk}],
    document: {},
    metadata: {
        published: boolean
        // method specific properties
    }
}

In doing so, add the optional boolean property published to the DidMetadata type.