perf: cache process.env value to increase performance

[RomainLanz]

Hey! 👋

This PR add a utility to retrieve the NODE_ENV & APP_KEY value from process.env.
This is usefull since accessing process.env has a huge cost in term of performance.

I'm also refactoring some HealthCheck functions and the ExceptionHandler to inject those value instead of refering them directly. This help us to create test and also remove possible side-effect.

[thetutlage]

This is nice. How about also improving the Env provider?

• We can keep the env values inside in-memory cache vs reading it everytime from the process.env.
• Also expose another method called env.clearCache(), which clears the in-memory cache

[RomainLanz]

Yes, we can make those changes to the Env provider.
We also need to check other modules if they directly access process.env!