add api-viewer source

add api-viewer source

so that we can reuse it across products
source was 'pbs' since that had the most features (http upgrade check)

a few changes to combine pve/pbs/pmg:
* use an optional 'cliusage' function it it exists to determine CLI usage
* check allowtoken for undefined to see if it is allowed or not
* use 'pmxapi' instead of pbs/pmg/pveapi
* rename all occurrences of 'pve' to 'pmx'

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