Xpay: use headercount as block height refering point

[Lagrang3]

Xpay gets the block height from the block_added notification.
In certain situations, the backend may be out of sync and in that case payment attempts will fail with
incorrect_or_unknown_payment_details if we are too far in the past.
A possible solution is to use the headercount as the correct block height.
There is no notification that we can use to fetch headercount, so we add as an alternative an additional
RPC call to getchaininfo at the start of the payment to update our internal block height.

Addresses issue #8612.
It uses a similar approach to #7190.

• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.