Add support for signaling an external process via pid file #430

Job	Run time
build-images	1m 8s
cache-deps (linux)	26s
build (linux)	46s
unit-test (ubuntu-22.04)	42s
unit-test (macos-latest)	27s
lint (linux)	46s
Build matrix	5s
integration-tests (change-entry-test.sh)	3m 28s
integration-tests (restore-entry-test.sh)	3m 26s
integration-tests (run-go-test.sh)	1m 48s
integration-tests (run-mysql-test.sh)	1m 46s
integration-tests (run-postgres-test.sh)	1m 41s
	16m 29s

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