implement signMessage on Solana TurnkeySigner

[montycheese]

Summary & Motivation

• Give sign message functionality to solana turnkey wallets

How I Tested These Changes

• Manually tested using the unit test and hard coded api keys

Did you add a changeset?

• yes
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[andrewkmin]

it would be great to be able to verify the signature on the spot

[andrewkmin]

lgtm! thanks for making this PR 🙏

one tiny thing to call out is that some signMessages implementations (e.g. on solana web3) are expected to return bytes/Uint8Array

[montycheese]

lgtm! thanks for making this PR 🙏

one tiny thing to call out is that some signMessages implementations (e.g. on solana web3) are expected to return bytes/Uint8Array

I can return a byte array instead of string to be more generic?

[r-n-o]

lgtm! thanks for making this PR 🙏
one tiny thing to call out is that some signMessages implementations (e.g. on solana web3) are expected to return bytes/Uint8Array

I can return a byte array instead of string to be more generic?

Yup! That'd be perfect. Also, if we make this change, then I believe the signMessage function should accept bytes as input rather than string? If you want to keep the convenience of signing utf8-strings then maybe a separate signText function would make sense?