Prepare to go monorepo - starting with backend

[m5nv]

Currently code that should be shared between gateway-server and resource-server is not being shared. Instead it is getting duplicated for lack of structure. The task is to move existing "proxy-server" and "crud-server" into a "backend" folder. And move common code into "backend/lib".

Acceptance Criteria

• All existing backend test cases should work with identical results before the reorg
• Code from two different lib folders should be merged into a single lib folder
• Duplicated code in resource-server and gateway-server should be consolidated where applicable

[m5nv]

Completed with this commit.