Add CORS headers to Mock API

[bzurkowski]

According to the solution proposed in #20 (review), we should allow all CORS origins in OpenRCA API to enable seamless communication from the UI. The Flask server should be configured with a proper CORS policy. Consider using a plugin such as Flask-CORS or try to modify the headers manually.

First, let's test the fix in the Mock API. Second, we will apply a similar patch to the core repository.

Some useful materials:

• https://medium.com/@baphemot/understanding-cors-18ad6b478e2b#:~:text=Cross%2DOrigin%20Resource%20Sharing%20(CORS)&text=CORS%20is%20a%20mechanism%20which,a%20different%20sub%20domain%20(eg.
• https://www.youtube.com/watch?v=mdQvEqqdFEI

[bzurkowski]

Closed by #41.