Allow catalog json to be served from GitHub and other

[astares]

Set the catalog source URL to a JSON that is provided by a GitHub location
and Pharo will fail to load it as currently only json mimetype is supported.

We should not restrict to also allow other like text/plain.

One can try out using the given URL:

https://raw.githubusercontent.com/astares/pharo-catalog/main/build/catalog.json

[astares]

To test try

CatalogSettings catalogSourceURL: 'https://raw.githubusercontent.com/astares/pharo-catalog/main/build/catalog.json'.

CatalogProvider default reset.
CatalogBrowser open

without and with the fix