Allow access transformers for forks

[yannicklamprecht]

Seems like the issue got lost here.
Too keep better track of it I'll open it here.

Request:

Add a possibility to declare AT's in forks of paper.

[TheShermanTanker]

Sorry I haven't had time to work on the enhancement for quite a while, I will try to return to paperweight development soon

[MiniDigger]

I wrote a thing that can apply ATs to source (pending cleanup): https://github.com/PaperMC/paperweight/blob/softspoon-v2/paperweight-lib/src/main/kotlin/io/papermc/paperweight/tasks/softspoon/ApplySourceAT.kt

if there is interest, that can be ported by somebody and used for fork ATs