[RFC] diff: add `diff.<driver>.process` for external hunk providers #28286

Job	Run time
config	6s
fuzz smoke test	1m 45s
win build	5m 11s
win+Meson build	4m 8s
static-analysis	6m 6s
documentation	7m 0s
sparse	3m 40s
rust-analysis	1m 32s
osx-reftable (macos-14)	28m 29s
osx-meson (macos-14)	45m 10s
osx-clang (macos-14)	6h 0m 17s
osx-gcc (macos-14)	33m 10s
linux-leaks (ubuntu:rolling)	41m 44s
win+VS build	0s
linux-asan-ubsan (ubuntu:rolling)	1h 1m 0s
linux-musl-meson (alpine:latest)	18m 0s
almalinux-8 (almalinux:8)	19m 52s
linux-sha256 (ubuntu:rolling)	19m 15s
debian-11 (debian:11)	25m 12s
linux-reftable (ubuntu:rolling)	27m 48s
linux-breaking-changes (ubuntu:rolling)	18m 6s
linux32 (i386/ubuntu:focal)	6h 0m 3s
linux-meson (ubuntu:rolling)	25m 5s
linux-reftable-leaks (ubuntu:rolling)	46m 34s
fedora-breaking-changes-meson (fedora:latest)	16m 23s
linux-TEST-vars (ubuntu:20.04)	27m 38s
win+VS test	0s
win+Meson test (3)	44m 50s
win+Meson test (8)	8m 44s
win+Meson test (7)	10m 6s
win+Meson test (9)	7m 25s
win+Meson test (0)	7m 29s
win+Meson test (1)	8m 35s
win+Meson test (2)	10m 47s
win+Meson test (4)	9m 18s
win+Meson test (5)	7m 40s
win+Meson test (6)	11m 29s
win test (0)	10m 55s
win test (7)	8m 4s
win test (8)	6m 59s
win test (6)	10m 5s
win test (4)	58m 15s
win test (3)	11m 21s
win test (9)	6m 39s
win test (1)	7m 43s
win test (2)	7m 5s
win test (5)	11m 30s
	1d 0h 28m 13s

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