Add asset support with hardware wallet signing

[TronBlack]

Ravencoin supports assets. This version of electrum currently supports Trezor, but does not support assets.

Trezor may or may not support generic signing of unrecognizable transaction types.

Adding any Hardware wallet (Trezor, KeepKey, Ledger, or Other) support for asset transaction signing. There will be additional proposals later for adding multi-sig signing of asset transactions once P2SH for assets has activated on the Ravencoin network.

Criteria:

• Add support for at least one hardware wallet for asset transactions
• Asset transaction UTXOs can be obtained from API, or RPC calls to Ravencoin node.
• Include Unit tests and functional tests

[RavencoinFoundationTron]

amount=USD$5000.00 paid_in=RVN status=open