rpc, evm: secure tx signing

[fedekunze]

Closes: #XXX

Description

Enable eth_sendTransaction and eth_sign rpc endpoints without passing the private key to the JSON-RPC service and relying solely on the keyring.

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[xlab]

Hello. This conditional signing makes it different from the previous codebase we had. And introduces discrepancy with many ethermint-based codebases e.g. informalsystems/hermes#1295 (review) where we made our Inj chain compatible.

And actually in my opinion this can lead to very shady bugs when one expects always a keccak, but if input matches 32 bytes (and IS NOT keccak), it will implicitly be omitted. I never saw that in other projects (cosmos / ethereum related).

[sunnya97]

Ack