Update credential definition methods to not generate id based on credential definition values

[TimoGlastra]

See #4 for general context and possible approaches

As the identifiers can now be any URI, we should update the methods in the AnonCreds library to not generate the id values, but rather allow the user to generate the IDs themselves based on the AnonCreds method they're using.

• Update the credx_create_credential_definition ffi method based on the chosen approach
  • remove origin_did as parameter
  • based on the chosen approach, add a cred_def_id parameter or not
• Update the create_credential_definition issuer.rs method based on the choses approach
  • remove origin_did parameter
  • based on the chosen approach, add a cred_def_id parameter or not
  • remove the generation of the cred_def_id based on the credential definition values
  • remove the logic of extracting the seq_no from the schema for the schema_id property in the cred_def, but rather use the schema.id directly for the schema_id property.
• If the frist approach is taken, we need to update all validation logic and make the cred_def_id optional (but only in some cases). This requires some refactoring probably