feat: add min_capacity_sat config value and switch

- add config value min_capacity_sat that will replaces the magic value
  min_effective_htlc_capacity = AMOUNT_MSAT(1000000)
- add config switch min_capacity_sat

wallet/test/test_utils.c

@@ -22,4 +22,5 @@ const struct config test_config = {
 	.rescan = 30,
 	.max_fee_multiplier = 10,
 	.use_dns = true,
+	.min_capacity_sat = 1000,
 };