add `alg` entry to `Issuer` type to support multiple signature schemes

[Mircea Nistor]

ES256K-R is hardcoded as the only supported algorithm which limits the use to only secp256k1 keys with custom verification.

The Issuer type should define the appropriate algorithm for the credential they need to issue.

Steps to resolve

1. add alg field to Issuer type (

   did-jwt-vc/src/types.ts

   Line 38 in 9b7b9cf

    export interface Issuer { 

   )

2. This issuer algorithm should then be passed to createJWT instead of the hardcoded JWT_ALG
   

   did-jwt-vc/src/index.ts

   Line 23 in 9b7b9cf

    alg: JWT_ALG 

   
   and
   

   did-jwt-vc/src/index.ts

   Line 35 in 9b7b9cf

    alg: JWT_ALG 

3. Add tests for ES256K and Ed25519