lxd/migrate: Metadata() push mode
lxd/migrate: Metadata() push mode

The migrationSink struct gains a Metadata() method. We will use it to send the
secrets back to the client as an operation.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@canonical.com>

hallyn/lxd,Skarlso/lxd,jsavikko/lxd,stgraber/lxd,techtonik/lxd,stgraber/lxd,lxc/lxd,atxwebs/lxd,georghopp/lxd,lxc/lxd,Skarlso/lxd,ollie314/lxd,atxwebs/lxd,lxc/lxd,atxwebs/lxd,mjeanson/lxd,lxc/lxd,chrisglass/lxd,pcdummy/lxd,erikmack/lxd,stgraber/lxd,hallyn/lxd,ollie314/lxd,hallyn/lxd,pcdummy/lxd,atxwebs/lxd,mjeanson/lxd,chrisglass/lxd,erikmack/lxd,georghopp/lxd,akshaykarle/lxd,stgraber/lxd,mjeanson/lxd,lxc/lxd,lxc/lxd,ollie314/lxd,pcdummy/lxd,Skarlso/lxd,georghopp/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,jsavikko/lxd,hallyn/lxd,erikmack/lxd,akshaykarle/lxd,hallyn/lxd,stgraber/lxd,jsavikko/lxd,chrisglass/lxd,hallyn/lxd,Skarlso/lxd,akshaykarle/lxd,techtonik/lxd,techtonik/lxd