Store URLs In Image Artifact

[collindutter]

• I have read and agree to the contributing guidelines.

Is your feature request related to a problem? Please describe.
ImageArtifact only supports bytes values. Often times APIs accept URLs as the image format.

Describe the solution you'd like
Some way for me to pass URLs to things like Image Variation Drivers

Describe alternatives you've considered
Cramming URLs into Image Artifacts and relying on the to_text to just work out.

Additional context
Requirement came from trying to implement https://bria-ai-api-docs.redoc.ly/tag/Image-Generation#operation/reimagine in https://github.com/griptape-ai/griptape-bria