config+lnd: add wallet-unlock-password-file option

In automated or unattended setups such as cluster/container
environments, unlocking the wallet through RPC presents a set of
challenges. Usually the password is present as a file somewhere in the
container already anyway so we might also just read it from there.

sample-lnd.conf

@@ -254,6 +254,11 @@
 ; BE USED ON MAINNET.
 ; noseedbackup=true
 
+; The full path to a file (or pipe/device) that contains the password for
+; unlocking the wallet; if set, no unlocking through RPC is possible and lnd
+; will exit if no wallet exists or the password is incorrect
+; wallet-unlock-password-file=/tmp/example.password
+
 ; Removes all transaction history from the on-chain wallet on startup, forcing a
 ; full chain rescan starting at the wallet's birthday. Implements the same
 ; functionality as btcwallet's dropwtxmgr command. Should be set to false after