Prover nodes are tracked, and their work is signed

[justinfrevert]

There should be some way to register prover nodes, put stake to their work, and have them sign off on their proof submissions.

Here are my thoughts towards implementation

• Require key in subkey/local keystore
• Prover signs the tx submission using the local keystore
• New storage map in the pallet which tracks the current prover nodes. The proof submission extrinsic will then only accept submissions from those prover nodes in the list.