config: allow bool values for repo cert

This change allows certificates.<repo>.cert configuration to accept
boolean values in addition to certificate paths. This allows for
repositories to skip TLS certificate validation for cases where
self-signed certificats are used by package sources.

In addition to the above, the certificate configuration handling has
now been delegated to a dedicated dataclass.

Co-authored-by: Celeborn2BeAlive <laurent.noel.c2ba@gmail.com>
Co-authored-by: Maayan Bar <maayanbar13@gmail.com>