Openchannels: new command for batched/coin-controlled channel opening

[bitromortac]

Introduces a new command openchannels with the intent of batching channel opens and implementing coin control, meaning one can select the UTXOs to spend from. Batching is implemented using LNDs support for PSBTs, so there is no external wallet required. The command expects a comma-separated list of public node ids as a minimal input. UTXOs and amounts can be specified as a comma-separated outpoint string.
The command will perform connection checks automatically in the setup phase. Once the setup phase is passed, channel opening is initiated. When everything goes well, before final signing and broadcasting, the user is asked to confirm the channel opening. Anchor reserve outputs are created/respected automatically.

The command has several modes of operation for specifying amounts and utxos:

• Specifying UTXOs defines a budget
• No amount is given: spend all UTXOs and distribute them equally to the channels - no change created
• Specify a total amount and no UTXOs: all UTXOs will be spent - no change created
• Specify amounts for channels and no UTXOs: all UTXOs will be spent:
  • (relative amounts) amounts are smaller than 100: relative amounts are rescaled to fit all UTXOs - no change created
  • (absolute amounts) amounts are larger than 100: if sum of amounts is smaller than the UTXO budget, change is created, if sum of amounts is larger than UTXO budget, the amount is rescaled such that no change is created

The user can decide if the opening should be private (unannounced) or public (announced).

TODO:

• Insert a timer to track the elapsed time
• Handle anchor reserves
• Decode PSBT for crosscheck
• Handle nested P2WKH inputs
• Reproduce LND fee estimation bug
• Report bug to LND repo
• final testing and code cleanup
• Implement a new command for UTXO locking/listing (in another PR)

[bitromortac]

Instead of maintaining an internal way of batching channels we could maybe use lightningnetwork/lnd#5356 .