feat: adds a little cli we can use for testing/demo to execute the needed operations

[cylewitruk]

Description

This just adds a little CLI we can build on for tasks needed for the demo (in the immediate future). Maybe we find it useful and want to keep some sort of CLI, in which case we should rename it :)

Changes

• Creates a bin directory and moves main.rs there but renamed as sbtc_signer.rs.
• Adds demo_cli.rs as a bin
• Added both bins to the Cargo.toml

Testing Information

Checklist:

• I have performed a self-review of my code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[cylewitruk]

Closing this as it's now out-of-date. Most of the important things have already been PR'd, and the CLI itself is pending in PR #825 .