Validation on resolution of PIDs may indicate errors in record due to non-resolvable fields

**Is your feature request related to a problem? Please describe.**

Resolving a PID with e.g. `/api/v1/pit/pid/21.11152/474a4b1c-de93-4d4a-b33d-1d32d63baf4b?validation=true` will fail because the record will contain a URL field.

This is because the URL field is currently considered interesting, and is returned by the HandleProcotol wrapper in the Typed PID Maker.

**Describe the solution you'd like / consider**

Either

a) make sure such fields are never returned
b) make sure such fields (which are not resolveable but part of the provider) are ignored while validation (at least when validating-on-resolution)

To be honest, b) sounds like it would complicate things. But we will need to discuss it.

**Additional context**

Searching for the PID will (as soon as the according branch is merged, which will soon be the case) lead you to a test case which can easily be modified to reproduce and analyze the issue.

Start with the PID resolution endpoint and the HandleProtocol wrapper in the Typed PID Maker, then locate the searching-PID test case mentioned in the issue and modify it to reproduce the URL-field failure. Compare whether non-resolvable provider fields should be excluded or ignored during validation; done means the intended behavior is agreed and covered by a regression test.

Written by the indexing model from the issue text.