Add correct payload to SDJwtVcInstance

[cre8]

When decoding an sd-jwt-vc credential with

const sdjwtvc = await sdjwt.decode(credential.credential as string);
console.log(sdjwtvc.jwt!.payload!.jti);

it will throw an error that jti comes from an index signature. It would be correct that we pass SdJwtVcPayload to the decode function so it knows that the default values from a JWT can be set.

Current solution is to cast it manually like console.log((sdjwtvc.jwt!.payload as SdJwtVcPayload).jti);

According to the SD-JWT-VC spec, jti is not a known value, but according to the JWT spec it is. Should we also add all values that are in a JWT also to the payload of a Sdjwtvc? Of course marking them all as optional.

[lukasjhan]

@cre8 I think it would be better to include the JWT spec. It would be inconvenient to use it by casting.