Add activity heartbeat sample. #43

Job	Run time
build-lint-test (ubuntu-latest, 3.2)	1m 9s
build-lint-test (ubuntu-latest, 3.4)	1m 9s
build-lint-test (macos-13, 3.2)	1m 33s
build-lint-test (macos-13, 3.4)	1m 25s
build-lint-test (macos-latest, 3.2)	49s
build-lint-test (macos-latest, 3.4)	57s
	7m 2s

Provide feedback