issuer-adapter: credentials should be bound to their holder

[llorllale]

The Issuer Adapter is not binding credentials to the holder: https://github.com/trustbloc/edge-adapter/blob/836557e6dced9285551dde2c0f806eb1274fea15/pkg/restapi/issuer/operation/operations.go#L801-L815

This results in "Bearer Credentials" with a weak Holder-VC binding.

The Issuer Adapter should receive the identifier the user would like to use as credentialSubject.id. This new DID would be delivered over the existing didcomm connection between the wallet and the issuer adapter, which is already an authenticated channel.