add fully dynamic observer triggering #13

Job	Run time
check-bevy-internal-imports	5s
build (windows-latest)	10m 2s
build (ubuntu-latest)	4m 28s
build (macos-latest)	4m 44s
check-doc	11m 38s
check-missing-examples-in-docs	32s
ci	4m 33s
miri	9m 42s
run-examples-macos-metal	5m 12s
toml	2m 4s
typos	9s
check-missing-features-in-docs	36s
markdownlint	1m 4s
check-compiles	6m 36s
check-compiles-no-std	49s
build-wasm-atomics	1m 58s
build-wasm	1m 38s
msrv	1m 45s
	1h 7m 35s

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