config: expose distinct accept-amp flag

This mirrors the accept-keysend flag, but also permits users to
eventually toggle off keysend separately from AMP.

sample-lnd.conf

@@ -375,6 +375,10 @@
 ; automatically. [experimental]
 ; keysend-hold-time=true
 
+; If true, spontaneous payments through AMP will be accepted. Payments to AMP
+; invoices will be accepted regardless of this setting.
+; accept-amp=true
+
 ; If set, lnd will use anchor channels by default if the remote channel party
 ; supports them. Note that lnd will require 1 UTXO to be reserved for this
 ; channel type if it is enabled.