[RFC] diff: add `diff.<driver>.process` for external hunk providers #28487

Job	Run time
config	9s
fuzz smoke test	1m 5s
sparse	3m 28s
win+Meson build	4m 5s
win build	4m 50s
static-analysis	5m 19s
rust-analysis	1m 30s
documentation	7m 35s
osx-gcc (macos-14)	2h 45m 38s
osx-reftable (macos-14)	41m 33s
osx-clang (macos-14)	2h 13m 54s
osx-meson (macos-14)	50m 29s
linux-breaking-changes (ubuntu:rolling)	24m 54s
linux-asan-ubsan (ubuntu:rolling)	1h 10m 48s
linux-reftable (ubuntu:rolling)	28m 17s
fedora-breaking-changes-meson (fedora:latest)	18m 17s
linux-leaks (ubuntu:rolling)	49m 31s
linux32 (i386/ubuntu:focal)	3h 3m 46s
linux-reftable-leaks (ubuntu:rolling)	47m 2s
linux-musl-meson (alpine:latest)	21m 25s
linux-sha256 (ubuntu:rolling)	29m 17s
linux-TEST-vars (ubuntu:20.04)	24m 7s
linux-meson (ubuntu:rolling)	26m 3s
debian-11 (debian:11)	22m 31s
almalinux-8 (almalinux:8)	23m 33s
win+VS build	0s
win+VS test	0s
win+Meson test (1)	8m 17s
win+Meson test (0)	5m 26s
win+Meson test (6)	12m 43s
win+Meson test (5)	10m 14s
win+Meson test (8)	10m 8s
win+Meson test (2)	7m 43s
win+Meson test (9)	7m 49s
win+Meson test (4)	12m 13s
win+Meson test (3)	57m 4s
win+Meson test (7)	8m 21s
win test (8)	7m 57s
win test (7)	8m 27s
win test (4)	8m 17s
win test (3)	13m 2s
win test (2)	7m 29s
win test (1)	7m 9s
win test (6)	19m 10s
win test (9)	7m 0s
win test (0)	10m 11s
win test (5)	59m 47s
	21h 17m 33s

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