Allow any values for the `format.ldp_vc.proof_type`

[TimoGlastra]

Currently the allowed values for the proof_type from ldp_vc format is restricted to a set of known types, namely:

    return [
      'Ed25519VerificationKey2018',
      'Ed25519Signature2018',
      'RsaSignature2018',
      'EcdsaSecp256k1Signature2019',
      'EcdsaSecp256k1RecoverySignature2020',
      'JsonWebSignature2020',
      'GpgSignature2020',
      'JcsEd25519Signature2020',
      'BbsBlsSignature2020',
      'Bls12381G2Key2020',
    ];

Is there a specific reason for this. As PEX doesn't handle the signature stuff, it could support any proof types and it would make the PEX library more flexible towards future signature suites that have yet to be defined.

[nklomp]

Reason is (if I remember correctly), that we have OpenAPI specs for the actual models (and endpoints for REST), allowing to limit this. But agree it probably make sense to either make this extensible or just use a string.

Will be updated ASAP