Header Oracle: have a way to be able to validate block data in history network

[kdeme]

Currently data is not validated as to be part of the canonical chain.
This means that in an offer/accept flow, as long as the content meets the structure of a block (header/body/receipts) it will be stored.

Another step is required to verify these blocks, but this requires header data. Eventually this could use the header gossip network to verify.
For now however, we will need another way, for example by downloading block headers and constructing the header accumulator already without the header gossip network. Or could use a full node endpoint to download the blocks (with only tx hashes) (yeah, sure, a bit silly for the Portal use case).

[kdeme]

As first step, this was implemented by requesting the epoch accumulators and using those in combination with the master accumulator to verify the headers (and related block bodies and receipts).

As a second step (and more efficient way), it was adjusted to provide headers with accumulator proofs, so that no additional request is needed to verify them. All the node needs is the master accumulator. This is implemented in #1281