add certificate panel

add certificate panel

Again, initially copied from PVE but adapted so it can be
used by both. (PVE side still needs to be tested though.)

The 'nodename' property is optional (since on PMG we
currently don't expose them via the UI directly). Instead,
the certificate info URL is required and the 'uploadButtons'
need to be passed, which just contains the certificate
"name", id (filename), url, and whether it is deletable and
whether a GUI reload is required after changing it. If only
1 entry is passed, the button stays a regular button (that
way PVE should still look the same), whereas in PMG we have
a menu to select between API and SMTP certificates.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>