Follow-up: keep getAddOnPurchase endpoint support in Duo Usage page

What does this MR do and why?

This MR is a follow-up of Support multiple Duo add-on tiers in Duo manage... (!160561 - merged), which swaps usage of the getAddOnPurchase endpoint for the newer getAddOnPurchases. Here, we re-introduce getAddOnPurchase in parallel to getAddonPurchases, in order to gracefully degrade due, so we can keep multi-version compatibility.

Context: !160561 (comment 2025278675)

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