OSDU stub server for development and testing Open-ETP-Server on local environment

Currently local developing/testing of Open-ETP-Server in the multi-partition mode is quite painful, even if you have real OSDU environment you still need to make stubs for Azure Identity service which is not accessible externally. Using real environment also will lead to fill it with a lot of dirty data, part of them could be not consistent or corrupted during testing/developing new features by developers. Some developers making stubs in the code, some try to use tools like Postman for make server stub (but this functionality is limited in time in free version).

The idea is to implement simple HTTP server with OSDU services stubs that are required by Open-ETP-Server. As alternative, the same can be implemented as mock of HTTP client, which could be activated via some command line parameter, but this will require to add code with testing mode to 'openETPServer' executable. In my opinion, standalone application is better for this purpose.

I've already implemented prototype of OSDU server stub, please take a look MR !98 (merged). It's based on already used library WebSockets++ and does not require any new dependencies. By default it's disabled, need to pass CMake option BUILD_OSDU_STUB_SERVER. Implemented OSDU stub has ability to save its state to files, that allow to not lose data like created Entitlements and Storage records between sessions.

Please review and approve the MR if you find this useful.