from() method must check for existing contexts and check for .revoked

[dlongley]

The from() method needs to check for an existing context on key, and if present, that it is either a string matching the ed25519 2018, 2020, or multikey context ... or if its an array that one of those contexts is present.

Additionally, it must check that key.revoked is undefined (otherwise throw a revoked error).

[dlongley]

For example code, see: https://github.com/digitalbazaar/data-integrity/blob/v1.0.0/lib/DataIntegrityProof.js#L255-L272