stake-pool-cli: Sign messages before checking fee

[joncinque]

Problem

get_fee_for_message is failing because the message is not signed, so there's no blockhash in the message yet.

Solution

Sign the messages before getting the fee

[t-nelson]

Signing something that may not be submitted makes my skin crawl. Consider Message::new_with_blockhash

[joncinque]

Oh that's great, thanks for noticing that! I assumed there was no way to attach a blockhash to a message without signing, but in all honestly, I didn't even check.