add support for external ssh signing #8103

Job	Run time
build-linux-musl (nightly)	9m 4s
build (ubuntu-latest, nightly)	2m 8s
build-linux-musl (stable)	8m 54s
build-linux-arm (nightly)	5m 53s
build (ubuntu-latest, stable)	2m 20s
build-apple-x86 (nightly)	4m 58s
build-linux-musl (1.70)	3m 30s
build-linux-arm (stable)	6m 11s
build (ubuntu-latest, 1.70)	8m 2s
build-apple-x86 (stable)	1m 45s
build-linux-arm (1.70)	6m 16s
build (macos-latest, nightly)	7m 17s
build-apple-x86 (1.70)	5m 47s
build (macos-latest, stable)	3m 26s
Lints	39s
build (macos-latest, 1.70)	2m 21s
Changelog Test	10s
build (windows-latest, nightly)	6m 49s
udeps	7m 24s
build (windows-latest, stable)	6m 45s
build (windows-latest, 1.70)	5m 5s
	1h 44m 44s

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