ValidateMaintainers: support validating URL instead name

[jkroepke]

In prometheus-community, we decide to use the maintainer URL as link to the GitHub profile in order to allow maintainers to use the full name, in my case:

- name: Jan-Otto Kröpke
  url: https://github.com/jkroepke

I would like to ask, if the maintainer validator could support this pattern as well? In that case, I would disable the name check, instead the URL check should return 200.

Ref: prometheus-community/helm-charts#5083 (comment)