[RFC] diff: add `diff.<driver>.process` for external hunk providers #28084

Job	Run time
config	9s
fuzz smoke test	1m 24s
win+Meson build	8s
win build	8s
sparse	8s
osx-clang (macos-14)	8s
rust-analysis	8s
osx-reftable (macos-14)	8s
documentation	8s
osx-meson (macos-14)	8s
osx-gcc (macos-14)	8s
static-analysis	8s
win+VS build	0s
linux-reftable (ubuntu:rolling)	8s
linux-breaking-changes (ubuntu:rolling)	8s
linux-sha256 (ubuntu:rolling)	8s
linux-musl-meson (alpine:latest)	8s
linux-leaks (ubuntu:rolling)	8s
linux-reftable-leaks (ubuntu:rolling)	8s
almalinux-8 (almalinux:8)	8s
linux-TEST-vars (ubuntu:20.04)	8s
debian-11 (debian:11)	8s
linux-meson (ubuntu:rolling)	8s
linux-asan-ubsan (ubuntu:rolling)	8s
fedora-breaking-changes-meson (fedora:latest)	8s
linux32 (i386/ubuntu:focal)	8s
win+VS test	0s
win+Meson test	0s
win test	0s
	4m 37s

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