[feature]: sub-sat/vbyte precision for sendcoins

[starius]

Is your feature request related to a problem? Please describe.

I would like to use lncli sendcoins to send a transaction with a fee rate of 1.2 sat/vbyte, as this is currently an optimal rate. However, the current implementation only supports integer values for the fee rate, which means I can only choose between 1 sat/vbyte (too low, causing long confirmation times) or 2 sat/vbyte (overpaying by 66%).

Describe the solution you'd like

Ideally, the following command should work as expected:

lncli sendcoins --sat_per_vbyte 1.2

Currently, the sat_per_vbyte option in SendCoinsRequest is defined as an integer. To address this, I propose adding a new field, sat_per_kw, to SendCoinsRequest.

• Only one of sat_per_vbyte or sat_per_kw should be allowed in a request.
• Modify lncli to accept fractional fee rates, convert them to sat/kw, and send them as sat_per_kw.
• We can deprecate sat_per_vbyte.

This change would allow users to specify more precise fee rates without unnecessary overpayment.

[ziggie1984]

we already have a PR for that: #7454

[lightninglabs-deploy]

Consolidating related issues here. The following have been closed as duplicates:

• Allow fractional funding fee rates for lncli openchannel --sat_per_vbyte #7456 — Requests fractional sat/vbyte fee rates for lncli openchannel --sat_per_vbyte. Motivation: with ordinals filling the mempool at 2 sat/vbyte, setting 2.1 sat/vbyte allows priority over them while keeping costs low. Notes the field is currently uint64 and would need a float/double type limited to 1-2 decimal places.
• lncli openchannel explicit fee control #5252 — Requests explicit fee control for openchannel, noting that the default fee estimation can result in surprisingly high fees. Also suggests showing a fee estimate before committing, or making --sat_per_byte mandatory.

[myxmaster]

I would love to see this land, active PR is this: #10067