change .env logic and fix remaining funds

[HamdiBenK]

-Separate the .env file in 3 files '.env.local' '.env.testnet' & '.env.mainnet'
-getting rid of different variable for testnet & prod
-remove logic and if conditions in const.js & config.js
-remove mongoConnectoin sum and juste use mongo URL to connecto to the db
-fix bug in getReamining funds api
-add ".env.example"

[ksibisamir]

Environment Variables: You have done an excellent job of managing sensitive information such as API keys, client secrets, and other configuration details by using environment variables. This is a critical practice for maintaining the security of your application. However, make sure to also have a .env.example file that outlines all the environment variables required by the application. This helps other developers understand what they need to provide to get the application running.

[ksibisamir]

Removal of Commented Code: I've noticed that a lot of code is currently commented out, particularly around the MongoDB connection configuration and contracts. If this code is no longer necessary, I would recommend removing it to improve readability. If parts of this code are still useful for future reference, I suggest moving it into a separate reference document or saving it in commit history.

Explanation for Commented Code: If some parts of the commented code are still relevant, it would be helpful to add comments explaining why they are commented out and under what circumstances they might be uncommented.