Added support for parsing image url strings

This PR lands support for parsing a string representation of an image url into `ImageUrl`-instance.

Use cases:
  - Combine with `imageUrl.getTransformations()` to get the applied transformations from a given URL
  - Creates an easier workflow in situations where storing transformations, image identifiers etc is non-trivial
  - When you have a chunk of rendered HTML or similar and want to modify the imageUrls present - for instance if the private key has changed