Add a configuration method to indicate that all .env files should be optional

[MrDave1999]

Example:

new EnvLoader()
      .AddEnvFile(".env.example1", Encoding.UTF8) 
      .AddEnvFile(".env.example2", encodingName: "UTF-8") 
      .AddEnvFile(".env.example3")
      .AllowAllEnvFilesOptional()
      .Load();

When calling the AllowAllEnvFilesOptional method, the loader will not generate any error when it does not found the .env files (.env.example1, .env.example2, .env.example3).