Payment PropertyBag: Add generic getter, setter methods for convenience

[artfulrobot]

Overview

In beginning to work with the new PropertyBag, we're realising some more sugar functions would be helpful.

Before

The only way to access properties was by knowing the getter's name; no way to "get if set otherwise give me a default".

After

You can use generic getter() and generic setter setter(), optionally giving a default.

Comments

Need discussed in #16052

Note: writing the tests for this threw up some interesting cases, esp. around custom property names and aliased property names (which you're not allowed to use because the only code using PropertyBag should know better!)

[civibot]

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[eileenmcnaughton]

I just switched this to the rc as we need to fix what will otherwise be a regression in the rc using this. This is as discussed so MOP