[RFC] diff: add `diff.<driver>.process` for external hunk providers #28093

Job	Run time
config	8s
fuzz smoke test	1m 21s
win build	5m 7s
rust-analysis	1m 32s
static-analysis	6m 17s
win+Meson build	5m 28s
osx-gcc (macos-14)	6h 0m 17s
osx-clang (macos-14)	36m 45s
osx-meson (macos-14)	45m 18s
documentation	7m 35s
osx-reftable (macos-14)	26m 45s
sparse	3m 33s
linux-reftable (ubuntu:rolling)	24m 15s
linux32 (i386/ubuntu:focal)	6h 0m 2s
almalinux-8 (almalinux:8)	19m 42s
debian-11 (debian:11)	22m 39s
linux-reftable-leaks (ubuntu:rolling)	46m 37s
linux-breaking-changes (ubuntu:rolling)	28m 32s
linux-leaks (ubuntu:rolling)	47m 59s
fedora-breaking-changes-meson (fedora:latest)	21m 15s
linux-TEST-vars (ubuntu:20.04)	27m 52s
linux-sha256 (ubuntu:rolling)	25m 39s
linux-musl-meson (alpine:latest)	18m 51s
linux-meson (ubuntu:rolling)	25m 2s
linux-asan-ubsan (ubuntu:rolling)	6h 0m 4s
win+VS build	0s
win+VS test	0s
win test (5)	14m 55s
win test (1)	7m 50s
win test (8)	7m 10s
win test (7)	8m 33s
win test (3)	9m 44s
win test (6)	9m 2s
win test (2)	8m 55s
win test (9)	6m 49s
win test (0)	11m 26s
win test (4)	58m 9s
win+Meson test (4)	9m 2s
win+Meson test (0)	7m 44s
win+Meson test (3)	59m 39s
win+Meson test (5)	8m 27s
win+Meson test (2)	11m 7s
win+Meson test (7)	6m 34s
win+Meson test (1)	7m 36s
win+Meson test (6)	10m 58s
win+Meson test (9)	6m 51s
win+Meson test (8)	9m 21s
	1d 6h 8m 27s

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