Proposal: Support Linked Data Signature format for expressing WebID Profile keys

[dmitrizagidulin]

In addition to the 2008 Cert ontology (which only expresses RSA keys), Solid WebID Profile documents should support expressing keys and cryptographic material using the current Linked Data Signatures format.

This would yield the following benefits:

• Enables support of other key formats, such as the Ed25519 (useful for signatures), Curve25519 (useful for encryption), and EcdsaSecp256k1 keys (useful for deterministic hierarchical key generation, used by many blockchains)
• Enables the usage of the WebID Profile document for the purposes of W3C Verifiable Credentials
• Moves the WebID Profile document towards compatibility with the W3C DID (Decentralized Identifiers) standard.

Example RSA key using LD-Sig:

<#key1>
  <http://purl.org/dc/terms/created> "2017-09-23T20:21:34Z"^^<http://www.w3.org/2001/XMLSchema#dateTime> .
  <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <https://w3id.org/security#RsaVerificationKey2018> .
  <https://w3id.org/security#controller> <https://alice.solid.community/profile/card> .
  <https://w3id.org/security#publicKeyPem> "-----BEGIN PUBLIC KEY...END PUBLIC KEY-----\r\n" .