Allow for custom baseline sources

[quentinguidee]

Request

Currently, the baselines are fetched from https://bl.vx.arra.cloud/. Vertex should provide a way to add custom sources for custom baselines.

This can be solved easily by adding a list of URLs to fetch, have a default URL to fetch, and allow to add new URLs to this list.

Why is this needed?

• Avoid single point of failure: if the domain is unreachable for any reasons, the only current way to update Vertex is to download again from GitHub, which is not trivial.
• People may want to create their own baselines, or use their own URL to provide custom baselines.