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
  If updating one of our packages, you'll likely need to add a changeset to your PR. To do so, run pnpm changeset. pnpm changeset will generate a file where you should write a human friendly message about the changes. Note how this (example) includes the package name (should be auto added by the command) along with the type of semver change (major.minor.patch) (which you should set).

These changes will be used at release time to determine what packages to publish and how to bump their version. For more context see this comment.

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