[RFC] diff: add `diff.<driver>.process` for external hunk providers #28080

Job	Run time
config	8s
fuzz smoke test	1m 16s
rust-analysis	15s
documentation	15s
win+Meson build	15s
static-analysis	23s
sparse	22s
win build	15s
osx-gcc (macos-14)	15s
osx-clang (macos-14)	15s
osx-reftable (macos-14)	15s
osx-meson (macos-14)	15s
debian-11 (debian:11)	15s
linux-breaking-changes (ubuntu:rolling)	15s
linux32 (i386/ubuntu:focal)	15s
fedora-breaking-changes-meson (fedora:latest)	15s
linux-leaks (ubuntu:rolling)	15s
linux-asan-ubsan (ubuntu:rolling)	15s
linux-sha256 (ubuntu:rolling)	15s
linux-TEST-vars (ubuntu:20.04)	15s
linux-musl-meson (alpine:latest)	15s
almalinux-8 (almalinux:8)	15s
linux-reftable-leaks (ubuntu:rolling)	15s
linux-meson (ubuntu:rolling)	15s
linux-reftable (ubuntu:rolling)	15s
win+VS build	-1s
win+VS test	0s
win+Meson test	-1s
win test	-1s
	7m 21s

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