Description
Is your feature request related to a problem? Please describe.
I'm working on a tool that automates large Lightning payments using multi-path payments (MPP) by specifying multiple outgoing channels. Currently, the lncli estimateroutefee command (and the underlying API) only supports probing a single outgoing channel at a time via the --outgoing_chan_id flag. This makes it impossible to estimate the route fee for payments that require MPP, i.e., where the payment amount exceeds the capacity of any single channel but could be fulfilled by splitting across several channels.
Describe the solution you'd like
I would like estimateroutefee (and the underlying API) to support a new argument, e.g., --outgoing_chan_ids, which would accept a list of channel IDs. This would allow the fee estimation logic to consider multi-path payments using the specified set of outgoing channels, similar to how lncli payinvoice supports MPP with multiple --outgoing_chan_id flags.
Describe alternatives you've considered
Probing each channel individually with estimateroutefee, but this does not reflect the actual payment logic for MPP, and always fails for large payments that require splitting.
Attempting the payment directly and parsing errors, but this is inefficient and can result in "payment in transition" errors and stuck HTLCs.
Additional context
This feature would greatly improve the ability to pre-flight check large payments and avoid unnecessary payment attempts that are doomed to fail due to insufficient single-channel liquidity.
It would also help avoid stuck HTLCs and "payment in transition" errors by allowing more accurate route probing for MPP scenarios.