Add api to fetch signed metadata updates data prior to erasure frames generation

[Slesarew]

Just give some api so that tools could fetched signed network specs and metadata (same content that's erasure-encoded in QR video) directly for client-side encoding.

This is needed in particular for tools like Kampela where the payload should be encoded into device-signed (on top of authority certificate signature) NFC fountain, which could only be done client-side. Splitting into packets is done client-side as well, as packet size could be variable. Only signature ensuring untampered original update content is really needed here, but serving content with the signature seems trivial here at this point and would be quite convenient.