`field.Filter` does not require `type` but `Filter` type in PEX requires it

[jiyoonie9]

I'm submitting a ...
[x] bug report
[x] release request
[ ] question about the decisions made in the repository
[ ] question about how to use this project

Summary

Thanks for building this library! We are using it to select credentials and I ran across a small issue while writing tests against test vectors.

In one of my test vectors, I have a Input Descriptor whose filter is a const requiring a specific string. Not having type inside filter object seems to cause a type error, but filter should behave like a JSON schema, and JSON schema does not require type.

          "input_descriptors": [
            {
              "id": "7b928839-f0b1-4237-893d-b27124b57952",
              "constraints": {
                "fields": [
                  {
                    "path": [
                      "$.vc.credentialSubject.name",
                      "$.credentialSubject.name"
                    ],
                    "filter": {
                      "const": "Satoshi Tacomoto"
                    }
                  }
                ]
              }
            }
          ]

When I take the filter object and put it in a JSON Schema validator, the object with just the const key should be a valid JSON schema.

Link to validator

It appears type is nullable in the latest version of fieldV2 in this repo, but the latest pex-model released (v2.2.4) does not reflect this change

All that to say - would you be able to release a new version of @sphereon/pex-model so that we can have a nullable type field in filterV2 type?

Thank you!