Correct final_cltv handling in blinded paths

When paying a blinded path, we don't have a CLTV delta at each hop
available, but rather only a total CLTV delta for the entire
blinded path.

However, the onion format currently still requires that we specify
an `outgoing_cltv_value` for the final hop. As the sender, we don't
have a sensible value to put there, as we don't know which part of
the total CLTV delta belongs to the recipient.

The sender is instructed to use the values that are known to them
when setting `outgoing_cltv_value` for the final hop:
- The current block height.
- Any additional delta added to account for block propagation and
  improve privacy.

This change reflects the behavior of some implementations at the time
of writing.

Author: TheBlueMatt

04-onion-routing.md

@@ -262,6 +262,10 @@ The writer of the TLV `payload`:
       - MUST include the `blinding_point` provided by the recipient in `current_blinding_point`
     - If it is the final node:
       - MUST include `amt_to_forward`, `outgoing_cltv_value` and `total_amount_msat`.
+      - The value set for `outgoing_cltv_value`: 
+        - MUST use the current block height as a baseline value. 
+        - if a [random offset](07-routing-gossip.md#recommendations-for-routing) was added to improve privacy:
+          - SHOULD add the offset to the baseline value.
     - MUST NOT include any other tlv field.
   - For every node outside of a blinded route:
     - MUST include `amt_to_forward` and `outgoing_cltv_value`.