lncli: allow PSBT to be read from file
lncli: allow PSBT to be read from file

Fixes #5080.

The N_TTY_BUF_SIZE kernel parameter dictates how many characters can be
pasted into a terminal window. This cannot be circumvented by reading
the input in a different manner.
To avoid the problem fully, we instead allso allow the user to type in a
path to a text file that is read if it exists. That way the PSBT can be
as long as needed.

aakselrod/lnd,aakselrod/lnd