Add resource minio_s3_bucket_file

[aminueza]

Add a new resource to manage buckets file objects.

Some rules to follow:

• add a new tag in provider.go: minio_s3_bucket_file;

• add an object in payload.go file named as
  type S3MinioFile struct

• create a func in check_config.go to fill up the object structure:
  func S3FileConfig

• add a test file naming as minio_s3_bucket_file_test.go;

• update the documentation creating a new S3_BUCKET_FILE.md file;

• create a .tf file with an example in examples/bucket/bucket.tf;

[hongkongkiwi]

This would be great! I'm particularly interested in this.

[gmile]

Wasn't this done recently in a form of managing the object in #72?

[gabrik]

Is this supposed to be used also to retrieve the content of the file?
Because I was unable to find a way to do it.