Add explicit extension point to matching algo

[zkoch]

^^

[adrianhopebailie]

#492

[adrianhopebailie]

At the WG call on 11 May the @adrianba agreed to propose a different PR (to supersede #492).

[marcoscaceres]

Postponing until after CR.

[ianbjacobs]

@marcoscaceres and @adrianba, would you mind providing rationale for the postponement? Thank you,

Ian

[marcoscaceres]

There has been no movement on it for many months and it's unstable for the purpose of CR. Also, I think we can solve it in the Handler spec, because basic-card handlers and proprietary handlers can, theoretically, already do everything without needing this "hook".

[marcoscaceres]

s/unstable/untestable... autocorrect 🤦‍♂️

[marcoscaceres]

Closing this, as I think we can easily add it when we actually need it (i.e., once we pivot back to payment handler and work out exactly, if anything, we need).