[lib] Rename generic types in action-utils to be more descriptive

Summary:
I personally prefer more descriptive generic type names vs single letter (eg T) generic names. Specifically, I found renaming the generic names in action-utils helped me make sense of what was going on.

I realize this is an "opinionated" change and using single letters for generic type names is common... so very open to abandoning this diff if others disagree.

Test Plan: Flow

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: palys-swm, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4318