Feature request: Add support for Proration

[dwanjuki]

Is your feature request related to a problem?
When using Payment Plans and Proration Add Ons, the checkout page level cost text very briefly displays prorated fees on load. This is then replaced with the default payment plan's fees.

It is not clear what amount will be charged.

Describe the solution you'd like
Add support for proration in the Payment Plans Add On.

Additional context
This came up in ticket 518702 (Mods only)

[kimcoleman]

This is most likely a priority issue - where Proration is trying to adjust the price at the same priority as the Payment Plan is trying to filter the level price. We can move the Payment Plans filter to priority 5 and while this could introduce other bugs, is probably the best way to ensure that other price adjusting logic happens after the select plan is activated.