[RFC] diff: add `diff.<driver>.process` for external hunk providers #28050

Job	Run time
config	10s
fuzz smoke test	1m 46s
win build	5m 3s
win+Meson build	4m 28s
static-analysis	6m 32s
documentation	6m 54s
sparse	3m 42s
rust-analysis	1m 29s
win+VS build	0s
osx-clang (macos-14)	35m 38s
osx-reftable (macos-14)	26m 38s
osx-gcc (macos-14)	6h 0m 18s
osx-meson (macos-14)	46m 47s
linux-leaks (ubuntu:rolling)	49m 21s
linux-reftable-leaks (ubuntu:rolling)	47m 28s
linux-reftable (ubuntu:rolling)	27m 55s
linux32 (i386/ubuntu:focal)	6h 0m 2s
fedora-breaking-changes-meson (fedora:latest)	21m 49s
almalinux-8 (almalinux:8)	19m 52s
linux-sha256 (ubuntu:rolling)	25m 26s
linux-breaking-changes (ubuntu:rolling)	21m 44s
linux-meson (ubuntu:rolling)	24m 46s
linux-asan-ubsan (ubuntu:rolling)	1h 7m 30s
debian-11 (debian:11)	21m 49s
linux-TEST-vars (ubuntu:20.04)	23m 55s
linux-musl-meson (alpine:latest)	23m 25s
win+VS test	-1s
win test (2)	10m 38s
win test (4)	56m 44s
win test (0)	10m 2s
win test (8)	6m 55s
win test (7)	7m 53s
win test (5)	15m 15s
win test (9)	6m 27s
win test (1)	7m 50s
win test (3)	9m 14s
win test (6)	8m 40s
win+Meson test (6)	12m 9s
win+Meson test (2)	10m 4s
win+Meson test (7)	9m 52s
win+Meson test (1)	8m 44s
win+Meson test (5)	8m 37s
win+Meson test (8)	9m 13s
win+Meson test (3)	58m 41s
win+Meson test (9)	6m 56s
win+Meson test (0)	7m 29s
win+Meson test (4)	12m 5s
	1d 1h 17m 54s

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