Prevent signing from wrong key in multisig

[ebuchman]

Summary

When signing a multisig tx, you are required to provide the multisig address (--multisig) and the key you are signing with (--from), but there's no check that the key is actually part of the multisig. This makes it very easy to accidentally sign with the wrong key and only figure it out when you try to broadcast the invalid tx that includes a signature from a key thats not in the multisig.

Would be amazing if the tx sign command could somehow check if the key you are trying to sign with is actually part of the multisig. This would presumably require access to the pubkey of the multisig, which the user should have anyways, and it should then be straightforward to check that the pubkey associated with the --from is in the multisig.

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