WIP: Open Feign must support multiple interface inheritance

WIP Notice

I am currently working on this feature request across multiple repositories, so this this will be updated multiple times. Once finished, this notice will be removed. As soon as this notice gets removed, there will be a PR for spring-cloud-openfeign.

Problem

The current problem with OpenFeign is that one cannot inherit from multiple interfaces. And while the current contract,, i.e. single inheritance of annotated interfaces, should be supported in a backward compatitble way, I see no reason why one should prevent the user to inherit from multiple interfaces or marker interfaces even, and recursively so.

Describe the solution you'd like

Feign client interfaces should support multi-level, multiple inheritance.

Describe alternatives you've considered

There are no alternatives unless you come up with a better solution to the problem.

Additional context

I am currently working on a PR for the open feign project to incorporate such measures, meaning that the Contract.BaseContract functionality will be extended so that the IMO artificial restriction on inheritance will be removed altogether.

This will require no changes to spring-cloud except for updating the openfeign dependency.

Start by reviewing the OpenFeign Contract.BaseContract functionality mentioned in the issue and how this repository consumes the OpenFeign dependency. Check the current inheritance behavior and integration points. Done means Spring Cloud OpenFeign supports multi-level, multiple interface inheritance without breaking single-inheritance compatibility, with the dependency update identified.