add eth_getReceiptProof

[ncitron]

Adds a new method, eth_getReceiptProof. This method takes in a transaction hash as input, and returns both a receipt, and a merkle proof of the receipt rooted at the receipt root.

The main use case I have in mind for this is light clients that utilize an RPC to prove state. Right now, the most efficient way to verify a given receipt is to download every receipt in the block, reconstruct the merkle tree, and check that the root of it matches. This method can instead be used, and significantly reduces the amount of data the light client must consume.

Both Helios and Kevlar want to make use of this method, and I imagine other light clients would as well.

[ncitron]

@etan-status pointed out that there has been some work towards migrating the receipt tree to an ssz tree in Daneb. It's probably best that we hold off on this method until after this change. Once this is decided I will update the schema to reflect the new tree format.

[etan-status]

Context for SSZ:

• https://eips.ethereum.org/EIPS/eip-6466

[etan-status]

And, for a demo that shows how the SSZ receipts would look like:

• https://eth-light.xyz (desktop only, broken on mobile)