returns blocks via hashes

[nbaztec]

What 💻

• Adds a concept of block hash for the test node. The block hash is computed as a keccak256 of the block_number and the tx_hash.
• Fixes the behavior of eth_getBlockByHash to now return a block via its hash. Previously the hash was matched with a transaction instead.
• Allows fetching for historical blocks over network if a fork source has been specified. The block is cached for any subsequent requests.

Why ✋

• Users of the test node can now expect the correct behavior when they call eth_getBlockByHash.
• Any missing historical blocks are now fetched and cached if a fork source has been provided. This aligns with the user expectations for this method call.
• The block specification of the test node is now more consistent with having an appropriate block hash, which is in line with user expectations.

Evidence 📷

Passing unit tests:

Notes 📝

Fixes #25

[nbaztec]

merged upstream