[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.