Add environment validation against schema

[castarco]

Is your feature request related to a problem? Please describe.
We can't enforce a schema "at once" over the loaded environment, instead, we have to check every variable one by one as we pass them to the "consumers".

This "okish" if you consume all the variables in one same place, but some people don't do that, so it can be clearly improved.

Describe the solution you'd like
Introduce a method allowing us to validate the loaded environment against a predefined schema in one single step.

[github-actions]

🥳 Thank you for opening your first issue for @coderspirit/safe-env ! 😀