PSBT channel funding support

[Jasonvdb]

Channels can now be funded visa external wallets initiating the process with the openChannelStream function and then progressing the funding state via the fundingStateStep function.

Steps:

1. Call openChannelStream to start the process. The onUpdate callback will be called each time the state progresses.
2. Use the fundingAmount and fundingAddress returned on the first callback result to create a PSBT in your external wallet with inputs (not yet signed).
3. Use fundingStateStep to verify the PSBT with LND.
4. Sign the PSBT inputs in your external wallet.
5. Use fundingStateStep to finalize the PSBT with LND.
6. onUpdate callback from step 1 will be called again with the updated channel status. onDone callback will be called once the tx has had all required confirmations.