[RFC] diff: add `diff.<driver>.process` for external hunk providers #28824

Job	Run time
config	8s
fuzz smoke test	1m 22s
win build	5m 9s
rust-analysis	1m 28s
sparse	3m 27s
documentation	6m 57s
win+Meson build	3m 6s
static-analysis	5m 55s
osx-reftable (macos-14)	26m 3s
osx-meson (macos-14)	39m 20s
osx-gcc (macos-14)	31m 59s
osx-clang (macos-14)	32m 1s
linux-reftable (ubuntu:rolling)	24m 42s
linux-sha256 (ubuntu:rolling)	24m 55s
linux-musl-meson (alpine:latest)	21m 7s
debian-11 (debian:11)	21m 47s
fedora-breaking-changes-meson (fedora:latest)	19m 0s
linux-reftable-leaks (ubuntu:rolling)	58m 3s
linux-TEST-vars (ubuntu:20.04)	23m 30s
linux-breaking-changes (ubuntu:rolling)	21m 8s
almalinux-8 (almalinux:8)	23m 9s
linux-leaks (ubuntu:rolling)	57m 25s
linux-asan-ubsan (ubuntu:rolling)	1h 10m 29s
linux32 (i386/ubuntu:20.04)	6h 0m 4s
linux-meson (ubuntu:rolling)	23m 57s
win+VS build	-1s
win+VS test	-1s
win+Meson test (3)	58m 35s
win+Meson test (4)	11m 42s
win+Meson test (1)	6m 54s
win+Meson test (8)	10m 13s
win+Meson test (7)	7m 23s
win+Meson test (2)	8m 8s
win+Meson test (5)	38m 16s
win+Meson test (9)	8m 44s
win+Meson test (6)	12m 3s
win+Meson test (0)	7m 28s
win test (0)	10m 40s
win test (7)	37m 17s
win test (1)	6m 57s
win test (8)	7m 12s
win test (6)	8m 17s
win test (2)	7m 54s
win test (5)	47m 7s
win test (9)	4m 40s
win test (4)	10m 24s
win test (3)	6m 53s
	20h 22m 56s

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