[RFC] diff: add `diff.<driver>.process` for external hunk providers #28480

Job	Run time
config	8s
fuzz smoke test	1m 30s
rust-analysis	1m 31s
win+Meson build	3m 55s
documentation	6m 56s
win build	5m 22s
static-analysis	4m 58s
sparse	3m 15s
osx-reftable (macos-14)	29m 44s
osx-meson (macos-14)	59m 54s
osx-clang (macos-14)	47m 46s
osx-gcc (macos-14)	44m 56s
linux-meson (ubuntu:rolling)	24m 56s
linux-leaks (ubuntu:rolling)	35m 35s
linux-musl-meson (alpine:latest)	21m 27s
almalinux-8 (almalinux:8)	14m 17s
linux-asan-ubsan (ubuntu:rolling)	26m 20s
linux-reftable (ubuntu:rolling)	23m 20s
debian-11 (debian:11)	2m 17s
fedora-breaking-changes-meson (fedora:latest)	13m 56s
linux-reftable-leaks (ubuntu:rolling)	12m 21s
linux-TEST-vars (ubuntu:20.04)	16m 34s
linux-sha256 (ubuntu:rolling)	23m 18s
linux-breaking-changes (ubuntu:rolling)	5m 20s
linux32 (i386/ubuntu:focal)	42s
win+VS build	0s
win+VS test	0s
win+Meson test (4)	11m 38s
win+Meson test (5)	10m 35s
win+Meson test (6)	12m 58s
win+Meson test (9)	9m 4s
win+Meson test (1)	7m 5s
win+Meson test (7)	7m 38s
win+Meson test (2)	7m 54s
win+Meson test (0)	6m 49s
win+Meson test (8)	9m 4s
win+Meson test (3)	33m 30s
win test (7)	8m 21s
win test (0)	9m 52s
win test (4)	7m 38s
win test (2)	7m 46s
win test (3)	9m 51s
win test (5)	59m 2s
win test (1)	7m 2s
win test (8)	7m 22s
win test (9)	6m 44s
win test (6)	20m 36s
	11h 30m 47s

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