[RFC] diff: add `diff.<driver>.process` for external hunk providers #28490

Job	Run time
config	8s
fuzz smoke test	1m 16s
win build	5m 8s
documentation	7m 6s
rust-analysis	1m 51s
win+Meson build	6m 4s
osx-reftable (macos-14)	26m 27s
osx-meson (macos-14)	35m 27s
sparse	3m 39s
static-analysis	6m 0s
debian-11 (debian:11)	15m 40s
osx-clang (macos-14)	9m 1s
linux-leaks (ubuntu:rolling)	35m 50s
linux-sha256 (ubuntu:rolling)	10m 28s
linux-TEST-vars (ubuntu:20.04)	24m 30s
linux-breaking-changes (ubuntu:rolling)	24m 47s
linux-asan-ubsan (ubuntu:rolling)	29m 24s
linux-musl-meson (alpine:latest)	23m 38s
osx-gcc (macos-14)	34m 42s
fedora-breaking-changes-meson (fedora:latest)	21m 53s
almalinux-8 (almalinux:8)	20m 19s
linux-meson (ubuntu:rolling)	24m 58s
linux-reftable-leaks (ubuntu:rolling)	35m 49s
win+VS build	0s
linux32 (i386/ubuntu:focal)	25m 16s
linux-reftable (ubuntu:rolling)	28m 34s
win+VS test	-1s
win test (2)	5m 25s
win test (0)	9m 53s
win test (6)	15m 21s
win test (1)	4m 24s
win test (7)	8m 13s
win test (4)	8m 5s
win test (3)	13m 3s
win test (8)	8m 3s
win test (9)	6m 45s
win test (5)	16m 18s
win+Meson test (4)	12m 29s
win+Meson test (7)	7m 33s
win+Meson test (5)	10m 52s
win+Meson test (0)	6m 35s
win+Meson test (8)	9m 47s
win+Meson test (1)	7m 6s
win+Meson test (9)	8m 23s
win+Meson test (2)	7m 22s
win+Meson test (3)	7m 46s
win+Meson test (6)	10m 12s
	10h 41m 29s

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