client/remote: add support to specify port number

client/remote: add support to specify port number

this adds the ability to add port numbers in the backup repo spec
as well as remotes, so that user that are behind a
NAT/Firewall/Reverse proxy can still use it

also adds some explanation and examples to the docs to make it clearer
for h2 client i left the localhost:8007 part, since it is not
configurable where we bind to

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>