[RFC] diff: add `diff.<driver>.process` for external hunk providers #28062

Job	Run time
config	11s
fuzz smoke test	1m 21s
win+Meson build	3m 19s
rust-analysis	1m 29s
sparse	4m 10s
static-analysis	6m 16s
osx-gcc (macos-14)	19m 31s
win build	5m 6s
win+VS build	-1s
osx-reftable (macos-14)	21m 10s
osx-meson (macos-14)	22m 36s
documentation	7m 8s
osx-clang (macos-14)	13m 44s
linux-sha256 (ubuntu:rolling)	22m 20s
linux-breaking-changes (ubuntu:rolling)	19m 18s
debian-11 (debian:11)	15m 13s
linux-musl-meson (alpine:latest)	19m 40s
fedora-breaking-changes-meson (fedora:latest)	16m 48s
linux-meson (ubuntu:rolling)	23m 43s
linux-reftable-leaks (ubuntu:rolling)	21m 58s
linux-TEST-vars (ubuntu:20.04)	24m 20s
almalinux-8 (almalinux:8)	19m 26s
linux-reftable (ubuntu:rolling)	21m 35s
linux-leaks (ubuntu:rolling)	22m 44s
linux32 (i386/ubuntu:focal)	32m 10s
linux-asan-ubsan (ubuntu:rolling)	22m 3s
win+VS test	0s
win+Meson test (7)	8m 58s
win+Meson test (5)	7m 40s
win+Meson test (4)	9m 10s
win+Meson test (0)	7m 36s
win+Meson test (9)	7m 39s
win+Meson test (2)	11m 17s
win+Meson test (3)	12m 19s
win+Meson test (6)	12m 11s
win+Meson test (8)	9m 17s
win+Meson test (1)	7m 38s
win test (3)	7m 52s
win test (1)	7m 48s
win test (8)	7m 36s
win test (0)	7m 41s
win test (6)	7m 44s
win test (2)	7m 58s
win test (4)	7m 40s
win test (5)	7m 35s
win test (9)	6m 15s
win test (7)	7m 32s
	9h 16m 44s

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