Allow catalog upload via URL instead of catalog_upload.json file

[ckadner]

Currently the the /catalog API allows to upload a JSON file to initialize the MLX asset catalog. This made it necessary to create a custom bootstrapper Docker image with a configmap with the JSON file contents, as well as a duplicate catalog_upload.json file for the Quickstart with Docker Compose.

A new /catalog/upload_from_url API endpoint that can take a URL to the catalog_upload.json file on the MLX Github repo would alleviate the need for a custom bootstrapper image for MLX deployment (an existing curl image without configmap would suffice) and the Quickstart could be run without having to clone the MLX repo, since the catalog upload would no longer require local files.