[Feature request] Create a flexible endpoint for documents

[stateandprove]

Current behaviour

There is currently an endpoint /rest/v1/id/xxx-xxx in the application that is responsible for a particular CRE instance. Such an instance has a unique identifier, which is a pair of three-digit numbers separated by a dash. Within the body of this instance there are links to other CREs and documents. If we want to get any linked CRE instance, we can easily get it using the above mentioned endpoint via its id.

Problem

This approach allows us to easily move from parent to child CRE instances and vice versa, but it doesn't include other many-to-many relationships, i.e. between CRE and linked document. It doesn't allow us to get a lot of important information, such as

• documents of a certain type for a given CRE
• all CREs linked to this document
  etc.

Feature Request

Using queryset parameters, we could easily solve both problems with the following requests:
/rest/v1/document/?CRE=xxx-xxx solves the first problem and
/rest/v1/document/?CRE=xxx-xxx&doctype=Standard solves the second.

So the request is to combine all documents within an endpoint and add the ability to filter document instances using the queryset parameter.
This would solve the problem mentioned in the #285 ticket and add other important functionality.