Allow different amounts when opening a channels

[D4nte]

Currently Alice and Bob can only commit the same amount into the channel.

We should provide an API to allow different amounts to be committed to allow more flexibility on the usage of the library.

As an extension, we should also allow making a payment as part of opening a channel, e.g.:

• Alice uses a 1BTC utxo
• Bob uses a 0.5 BTC utxo
• Alice pays bob 0.1BTC
• The channel balance once created is: Alice 0.9BTC, Bob 0.6BTC

This should be done by allowing custom outputs in the split transactions.