Proto: SCORU: Parse messages in CLI as strings

Context

The "send message" CLI command for SCORU currently expects messages to be hex-encoded when passed directly via the command-line.

Given the utility status of that CLI command and the overhead involved when trying to send example messages, it would be useful to allow users to submit messages in plain string format without the need to encode them.

https://tezos-dev.slack.com/archives/C02HSTL4D7H/p1649151635493899

Manually testing the MR

Checklist

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