Add provider selection logic, file provider and tests

[csatib02]

Overview

This PR contains a file provider, with tests, and implemented provider selection logic.

Continuation of issue: #1

Functionality

The provider selection works by getting it from an environment variable.
The file provider by default uses / as root path for file system.

The main function has been reworked:

1. Provider creation.
2. Extract the secrets path from environment variables.
3. Load secrets using the provider. (This gives back a slice of secret data, which contains a field for the secret itself, and another for the path.)
4. Create environment variables from the loaded secrets, and map them to the right environment variable key, using the struct's path field.

env.go has been created.
It encapsulates functionality extracted from the main package, providing a clean separation of concerns and promoting reusability. This approach also enhances testability.

[ramizpolic]

I left the final review on the overall logic. The flow makes sense now 👍

[ramizpolic]

Great work @csatib02, lgtm!