Detect the provider-metadata.json files by domain

[bernhardreiter]

A user wants to enter a domain and see if they are already offering CSAF documents.

If a simple backend is offered, (like #38) then adding code to detect the provider-metadata.json (PMD) s on the backend side could be helpful so the app could offer to take domain names.

The backend can utilise the https://github.com/csaf-poc/csaf_distribution code so that PMD detection is only implemented once.