ENH Fetch respositories.json from raw.githubusercontent.com

[emteknetnz]

Issue silverstripe/rhino#3

[GuySartorelli]

I don't think this is useful for any of the repos using it other than Rhino:

• silverstripe/cow: We do a composer update before running this each release anyway
• silverstripe/silverstripe-tx-translator: We pull this in fresh each time we use it
• bringyourownideas/silverstripe-maintainence: Only cares about the actual commercially supported modules, which we hardly ever touch
• silverstripe/github-issue-search-client: Already fetches the raw JSON file from GitHub
• silverstripe/module-standardiser: We pull this in fresh each time we use it
• silverstripe/.github: Already fetches the raw JSON file from GitHub
• silverstripe/api.silverstripe.org: Only cares about the actual commercially supported modules, which we hardly ever touch
• silverstripe/gha-ci: Already fetches the raw JSON file from GitHub

The only item this might be useful for is Rhino. So I'd suggest either we only do this for Rhino (in which case we don't do it in this repo at all), or we just leave rhino as it currently is for a bit and see if it gets annoying. I don't think it will get annoying to redeploy Rhino when we add new repos, because:

1. We don't update the list very often
2. Ideally we get Rhino to auto-deploy when PRs get merged at some point, which reduces the steps required to get the changes in

[emteknetnz]

Chatted offline, will proceed with this PR

[GuySartorelli]

We should ideally either use a global constant, or better yet have a property for the path set to 'https://raw.githubusercontent.com/silverstripe/supported-modules/main/repositories.json' by default but which unit tests set to the path of the raw file in this repo.
Or for the cleanest and most standard way, mock the request in the unit test.

That said, it's probably not worth doing another round of review for my perfectionism :p This works well enough.

[GuySartorelli]

Good enough to merge