[RFC] diff: add `diff.<driver>.process` for external hunk providers #27984

Job	Run time
config	8s
fuzz smoke test	1m 21s
win build	5m 23s
win+Meson build	2m 3s
osx-gcc (macos-14)	6h 0m 16s
osx-meson (macos-14)	22s
osx-reftable (macos-14)	37m 46s
sparse	3m 53s
osx-clang (macos-14)	6h 0m 18s
rust-analysis	1m 28s
documentation	3m 17s
static-analysis	5m 19s
linux32 (i386/ubuntu:focal)	6h 0m 5s
linux-asan-ubsan (ubuntu:rolling)	1h 9m 13s
linux-reftable (ubuntu:rolling)	23m 54s
debian-11 (debian:11)	22m 21s
almalinux-8 (almalinux:8)	20m 4s
linux-TEST-vars (ubuntu:20.04)	20m 56s
linux-sha256 (ubuntu:rolling)	16m 5s
linux-musl-meson (alpine:latest)	18s
fedora-breaking-changes-meson (fedora:latest)	57s
linux-leaks (ubuntu:rolling)	48m 13s
linux-meson (ubuntu:rolling)	1m 18s
linux-reftable-leaks (ubuntu:rolling)	46m 46s
linux-breaking-changes (ubuntu:rolling)	28m 3s
win+VS build	0s
win+VS test	0s
win+Meson test	-1s
win test (0)	13m 12s
win test (3)	1h 7m 19s
win test (2)	9m 4s
win test (4)	8m 46s
win test (1)	6m 17s
win test (6)	7m 35s
win test (9)	5m 42s
win test (7)	6m 41s
win test (5)	10m 49s
win test (8)	6m 53s
	1d 2h 22m 4s

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