Add a configuration method to set the environment variables provider

[MrDave1999]

The idea of this new function is that the consumer can create his own environment variables provider and inject it into a configuration method.

For example:

class CustomProvider : IEnvironmentVariablesProvider
{
     private Dictionary<string, string> _keyValuePairs = new Dictionary<string, string>();

     public string this[string variable] 
     {
          get => _keyValuePairs.ContainsKey(variable) ? _keyValuePairs[variable] : null;
          set => _keyValuePairs[variable] = value;
     }

     public IEnumerator<KeyValuePair<string, string>> GetEnumerator()
          => _keyValuePairs.GetEnumerator();

     IEnumerator IEnumerable.GetEnumerator()
          => this.GetEnumerator();
}

var envVarsProvider = new CustomProvider();
new EnvLoader()
         .SetEnvironmentVariablesProvider(envVarsProvider) // configuration method
         .Load();

string key1 = envVarsProvider["KEY1"];
string key2 = envVarsProvider["KEY2"];

When creating our own provider, the environment variables are not obtained from the current process, but from the custom provider (in this case it is a dictionary<string, string>).

Note: The IEnvironmentVariablesProvider interface is specific to the library.