KeyManager key_id bug

[edchapman88]

• IonAttestor.signing_key() takes an optional key_id argument

• The specified key is returned by looping over the JWK's returned by the read_signing_keys() method on the AttestorKeyManager trait.

• Currently the loop matches on the optional key_id field of the JWK struct.

• This JWK field is currently not populated when keys are written to file by the trustchain key manager.

• Instead, the query key_id should be matched against the compulsory id field associated with each key in the verificationMethod field of the DID Document.

• To implement this change, the associated key id's for each key in a DID Document must be maintained in the file system (eg. written to file during the did create operation)