Repo Certificate Authority field for private repos

[stefanha]

Public repos pass certificate verification using the system's trusted root certificates. Private repos may require a Certificate Authority that is not in the system's trusted root certificates.

Although some users may be able to add CAs system-wide, it is easiest and cleanest to allow setting CAs on a per-repo basis. This way no change system-wide change is necessary in order to use private repos with custom CAs.

The repo.yaml syntax can be extended with a CA string field so users can provide the CA together with the repository information.

@andreabolognani