signArc60Data takes an optional `scheme`, as proposed for ARC-60 in
algorandfoundation/ARCs#494 and already sent by use-wallet 5.1
(TxnLab/use-wallet#473) and Lute.

- ed25519 requests (no scheme, or "ed25519") go out exactly as before,
  with `signer` as an address.
- Any other scheme sends `signer` as base64 of the key, since a Falcon
  address is a hash of its key, and `scheme` beside it for the wallet
  to check. The response keeps both.
- verifySignature only handles ed25519. For another scheme it throws
  SIGN_DATA_VERIFICATION_UNSUPPORTED before the wallet is asked.
- signArc60Data no longer encodes the signer as an address after the
  wallet answers when it isn't verifying, which would have thrown away
  a Falcon signature.

Pera versions that don't sign Falcon ARC-60 yet reject these requests:
their wire schema caps `signer` at 128 characters.