[Design doc] P256 Signature Proxy Support

[p5150j]

We need to get a design doc started for Passkey support. This should follow the other design doc patterns.

Additions to design doc needed

• need a data flow diagram
• need a data map for legal teams
• .....

Design Doc Readme and Template: https://github.com/LibertyDSNP/meta/blob/main/DESIGN_DOCS.md

Design Doc Needs

• [x] Data flow diagram
  • [x] What information is stored and accessible where
• [x] Terminology
  • [x] Different Keys
  • [x] Signature Proxy Pallet?
• [x] Why section
  • [x] Ease of user using tokens
  • [x] Options for backup and recovery
  • [x] etc...
• [x] Spec on the passkey registration
  • [x] Security considerations
  • [x] What should be done on the client side JS and server side verification for safety
    • [x] eg: Never store seed phrase except in memory
    • [x] Always validate that the end user has had a chance to download the seed phrase
    • [x] Make sure 3rd party js cannot access seed phrase
• [x] Spec on how to form and submit transactions aka the on-chain spec that the code will complete
  • [x] Inputs
  • [x] Hashes
  • [x] Signature data for verification
  • [x] P256 Proxy should sign:
    • [x] Tx Params
      • [x] Proxy Auth signature
      • [x] Parent/root Account Id
      • [x] Call
    • [x] Nonce
    • [x] Mortality?
• [x] The new Signed Extension(s) and what they will do
  • [x] CheckNonce (Should use the nonce of the parent/root account)
  • [x] ChargeAssetTxPayment or ChargeTransactionPayment (Should use the balance of the parent/root account)
  • [x] Add / Remove from other Signed Extension lists around Capacity/Free Tx or filters
• [x] Example code for how to use this from a backend
• [x] Example code for how to use this from a frontend
• [x] Options if needed for discussion by the team
  • [x] Unsigned Extensions vs MultiSignature and wrapping Signed Extensions

Maybes

• [x] Should we make this more generic and allow use of keys from wallets not just generated keys?
• [x] Should we make MultiSignature support P256?
  • [x] Should we allow for P256 Accounts to hold tokens?
  • [x] Should we allow for P256 Accounts be MSA control keys? (Yes, but perhaps not now?)
• [x] Should we make a separate pallet for this?
  • [x] If so, can all the needed signed extensions also be in this pallet?

[aramikm]