api: add remote migrate endpoint

api: add remote migrate endpoint

entry point for the remote migration on the source side, mainly
preparing the API client that gets passed to the actual migration code
and doing some parameter parsing.

querying of the remote sides resources (like available storages, free
VMIDs, lookup of endpoint details for specific nodes, ...) should be
done by the client - see next commit with CLI example.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>