Allow to export/import outputs and image keys in raw format

[mahnunchik]

Allow to export/import outputs and image keys in raw (not encrypted) format.

[woodser]

importKeyImages() and exportKeyImages() use an unencrypted format.

exportOutputs() returns a hex string to be used with importOutputs().
This functionality depends on monero-wallet-rpc so additional support would first need to be added there.

[mahnunchik]

There are unencrypted versions of this function in wallet2.cpp file:

• wallet2::export_outputs https://github.com/monero-project/monero/blob/master/src/wallet/wallet2.cpp#L13066-L13085
• wallet2::import_output https://github.com/monero-project/monero/blob/master/src/wallet/wallet2.cpp#L13106-L13172

[woodser]

My recommended approach is to rename MoneroWallet's exportOutputs() to exportOutputsStr() and add a new function, exportOutputs(), which returns a map from output offsets to MoneroOutputWallet like wallet2::export_outputs(). importOutputs() would accept either a string or the map.

Only MoneroWalletFull would support the functionality. MoneroWalletRpc would throw an unsupported error if exportOutputs() is invoked.

The implementation would be in monero-cpp's monero_wallet_full.cpp + the necessary bindings to JavaScript + tests.