Rewrite Signer in Rust

[bhgomes]

Features

• Rewrote the signer server implementation in Rust with basic HTML/JS UI for password input
• Can now leverage tauri for simple/portable application bundling
• Added test server implementation

TODO

• Check that the HTTP/CORS interface matches with frontend (@Kevingislason @bhgomes)
• Use resolve_path to add a global directory to put the root_seed/private_transfer_pk/reclaim_pk. This will also require the ability to re-compute the keys when they are missing. (@bhgomes)
• Add zeroize and/or secrecy adapters for passwords (@bhgomes)

[Kevingislason]

Looks good!

re: Shumo's comment, I lean towards move batching logic to manta-api instead of the reverse, since idk we might eventually have different implementations of signer, and we could re-use that logic. But obvs not important now.

[bhgomes]

re: Shumo's comment, I lean towards move batching logic to manta-api instead of the reverse, since idk we might eventually have different implementations of signer, and we could re-use that logic. But obvs not important now.

👍 It'll probably all be in manta-rs anyway by the time this is really relevant.