New Transaction.GetInterchain and Block.GetInterchain call the prime-node
/api/v1/{transaction,block}/{id}/interchain endpoints, returning an
InterchainTrace {blockId, validatorBlocks, interchainTransactions}. Adds local
VerificationBlock / InterchainTransaction / InterchainTrace model types.
The block endpoint returns block id / prev / timestamp nested under a
"header" object with camelCase keys (blockId, dcId, prevId, prevProof,
timestamp) and a proof of just {proof}. The previous flat snake_case
Block fields never matched the response and always deserialized empty.
Add a BlockHeader struct, nest it in Block, and make Proof.Scheme
omitempty. Verified live against a dev chain.
