lsp_plugin: change extra_fee value type to u64

Blip25 https://github.com/lightning/blips/blob/master/blip-0025.md uses
u64 for the `amount_msat` in the `extra_fee`

Signed-off-by: Peter Neuroth <pet.v.ne@gmail.com>

Author: nepet

plugins/lsps-plugin/src/client.rs

@@ -445,7 +445,7 @@ async fn on_htlc_accepted(
 
     // Safe unwrap(): we already checked that `extra_tlvs` exists.
     let extra_tlvs = req.htlc.extra_tlvs.unwrap();
-    let deducted_amt = match extra_tlvs.get_tu64(65537)? {
+    let deducted_amt = match extra_tlvs.get_u64(65537)? {
         Some(amt) => amt,
         None => {
             warn!("htlc is missing the extra_fee amount");