Support configurable verifiers for VC and VP

Overview

The service will have new ENV variable containing a list of verifiers (array). Initially there will be only one verifier named TRAIN.

CREDENTIAL_VERIFIERS="TRAIN"

In case there will be more than one verifier, it will be comma separated strings

CREDENTIAL_VERIFIERS="TRAIN,TRAIN2"

Verifiers will implement a specific interface to handle verification, for example:

import 	"github.com/hyperledger/aries-framework-go/pkg/doc/verifiable"

type Verifier interface {
  VerifyCredential(ctx context.Context, vc *verifiable.Credential) error
  VerifyPresentation(ctx context.Context, vp *verifiable.Presentation) error
}

When the service starts all configured verifiers will be initialized and ready for use. When a VC or VP needs to be verified, the service will use the initialized verifiers to perform additional verification.

These functionalities will be developed in the Signer service, as all VC/VP verifications happen there.

Trusted Content Resolver

Here is the TCR service which implements the TRAIN validation details.

https://gitlab.eclipse.org/eclipse/xfsc/train/trusted-content-resolver