Add env selector to provider implementations

[ramizpolic]

The provider logic is broken into two places: a) for selecting secrets from ENV and b) for loading required secrets loaded from env.
We can add Valid(envKey, envValue string) bool method to providers which can be used to decide which provider to use when loading secrets.

The provider initialization logic should be also be adjusted so that we can loop/lookup available providers in order to test env-secret validity.

Further clarify and have proper planning meeting before working on this.