Add support for relaying with the vote extension

[gjermundgaraba]

Probably need to fetch and add the actual packets to the sidecar server response somehow.
And then propagate the packets through the vote extensions and finally submit them after updating the light client.

It seems a little wasteful that every validator is relaying the packets, so maybe there is a smarter way to round-robin it or in some way only have a few actually fetch packets.

[gjermundgaraba]

Closing this for now, as this needs some decision per #64