Create an exportable Typescript interface for Open Badges v3 (OBv3) type credentials #11

The current IVerifiableCredential interface exported from this library covers the general VC Data Model 2.0 spec. Which does not specify any details of what goes into a credentialSubject. It just defines the envelope.

Other specs have more details/requirements for what goes into a credentialSubject. The main one that we're using (both in LCW, and in the T3 *Author apps) is the OBv3 Spec.

The Task: Export an IOpenBadgeCredentialV3 interface that extends IVerifiableCredential currently defined in VCDM.ts.

• Place it in a separate file, src/OBv3.ts.

• Start generic (for example, you can start with just the credentialSubject.achievement and evidence definitions), and you can add more details in other PRs.

To help with this task, here is the OBv3 JSON Schema definition: https://www.imsglobal.org/spec/ob/v3p0/#json-schema -- see if there are "JSON Schema to Typescript Interface definitions" tools that can help with the translation.

Ideally, you'll want to comment each field definition and link to the corresponding section in the OBv3 spec, like the IVerifiableCredential interface does with the VC spec.