[RFC] diff: add `diff.<driver>.process` for external hunk providers #28040

Job	Run time
config	6s
fuzz smoke test	1m 43s
win build	4m 58s
win+Meson build	4m 33s
documentation	7m 17s
sparse	3m 54s
osx-meson (macos-14)	12m 47s
osx-clang (macos-14)	25m 14s
osx-gcc (macos-14)	25m 14s
rust-analysis	1m 28s
osx-reftable (macos-14)	25m 14s
static-analysis	5m 53s
linux-reftable-leaks (ubuntu:rolling)	25m 16s
linux-asan-ubsan (ubuntu:rolling)	24m 33s
linux-breaking-changes (ubuntu:rolling)	25m 15s
debian-11 (debian:11)	21m 39s
linux32 (i386/ubuntu:focal)	25m 14s
linux-musl-meson (alpine:latest)	23m 29s
linux-reftable (ubuntu:rolling)	21m 17s
linux-TEST-vars (ubuntu:20.04)	23m 30s
win+VS build	0s
linux-meson (ubuntu:rolling)	17m 56s
linux-leaks (ubuntu:rolling)	20m 4s
linux-sha256 (ubuntu:rolling)	20m 16s
fedora-breaking-changes-meson (fedora:latest)	12m 33s
almalinux-8 (almalinux:8)	12m 21s
win+VS test	0s
win test (8)	6m 45s
win test (6)	9m 9s
win test (9)	4m 44s
win test (0)	7m 46s
win test (1)	7m 11s
win test (2)	11m 8s
win test (5)	15m 44s
win test (3)	8m 59s
win test (4)	20m 7s
win test (7)	6m 3s
win+Meson test (3)	15m 8s
win+Meson test (0)	7m 16s
win+Meson test (1)	7m 27s
win+Meson test (4)	8m 9s
win+Meson test (2)	6m 52s
win+Meson test (6)	8m 24s
win+Meson test (5)	7m 11s
win+Meson test (9)	7m 39s
win+Meson test (8)	6m 20s
win+Meson test (7)	6m 19s
	9h 30m 5s

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