Make use of system-specific fields

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

For different PID records, we may want to have the URL field of the Handle System to be filled using a value of a different type. It may be the digitalObjectLocation for many, but for some it may be a landing page or so.

**Describe the solution you'd like**

When doing a request, I want to specify the URL field. Either in the JSON input, or as a URL parameter.

**Additional context**

- as an optional parameter, this would not be a breaking change
- the current way (having a fixed configuration) would stay as the default in order to not break functionality
- For other systems than the handle system, this may be ignored.
- This is somehow related to #253 , where I suggest to ignore URL fields. If we ignore them, the parameter should not be part of the usual attributes as input as well. If we allow it though and validation may ignore it, we could do it here, too. In order to not complicate things, **I propose a url parameter like "?copyRedirectUrlFrom=$attributePID"** or similar.

The issue names no files or tests; start by locating the request entry point and the current fixed Handle System URL configuration. Review issue #253 and determine how an optional URL-source parameter should interact with JSON input, validation, and other PID systems. Done means the configured default remains unchanged while the requested source can be selected without affecting unsupported systems.

Written by the indexing model from the issue text.