Add support for enabling vnc clipboard

[sei-aschlackman]

Feature Request
Proxmox 8.1 added clipboard support to the VNC console:

"Add clipboard support to the VNC console in the GUI. For now the feature cannot be enabled in the GUI and has to be manually enabled via API or CLI. After installing the SPICE guest tools, text can be copied from/to the guest clipboard using the noVNC clipboard button."

This can be enabled through the CLI with qm set <vm-id> -vga clipboard=vnc or through the API in the vm creation POST through the vga parameter with [[type=]<enum>] [,clipboard=<vnc>] [,memory=<integer>]

Describe the solution you'd like
Add an additional clipboard property to the existing vga argument that can be set to vnc to enable vnc clipboard support on a virtual machine.