Add new Payment.sendconfirmation api

Add new Payment.sendconfirmation api

This adds a new api Payment.sendconfirmation intended to be an equivalent of
Contribution.sendconfirmation.

The key thing this needs to be able to do is to load up all
related information to assign to the template - this is not all covered in this
PR & until it is functionally equivalent to AdditionalPayment::sendEmail
we can't switch over but since it's new functionality we should be able to
merge & keep working on it.

Note that there is discussion on the PR this relates to (#13330) about what should
happen if 'Payment.create' api accepts 'is_email_receipt'. I think the most logical
outcome is that receipts would go out if it caused the contribution to be completed
(ie. we pass is_email_receipt into completetransaction api).

However, I don't feel like that is 'settled' yet - separating into a second api
(Payment.sendnotification) means we will have 2 generic function which can be called
from multiple places in the code rather than 2 functions tightly tied to the form layer.
I think it's OK if it is 2 not one