EVM/Proxy: allow batching requests

Context

Some EVM Clients (Geth for example) support receiving multiple requests at once. Batch are described in the JSON-RPC specification. The order of responses are not specified, but I chose to set it to the same order than the requests.

These are actually necessary for indexers that use these extensively to fetch blocks and transactions.

Manually testing the MR

I've added two tests:

• one that checks a single request or a batch of one request are equivalent
• one that batch two requests and check the results are consistent

dune exec tezt/tests/main.exe -- --file evm_rollup.ml

Checklist

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