Accept plain OAuth2 Bearer Tokens, not base64-encoded

[llorllale]

I still believe that RFC6750 mandates the base64-encoding of the access_token in the Authorization header but after further research into it I've realized that many if not all vendors process the token in plain form (more precisely, in the same format, byte-for-byte, as was delivered in the access token response (RFC6749 section 4.1.4)).

Client components impacted by this change are:

• hub-kms
• edge-agent

https://github.com/trustbloc/hub-auth/blob/413a3517d5d5a373780c7f55d7f866456db5a31a/pkg/restapi/operation/operations.go#L806-L833