Add implementations for onShare and onSearchWeb #5057

Job	Run time
android-build	12m 50s
Test Flutter stable on ubuntu-latest	19m 11s
ios-build	16m 4s
web/samples_index config check	51s
Test Flutter stable on macos-latest	39m 41s
Test Flutter stable on windows-latest	25m 22s
Test Flutter beta on ubuntu-latest	17m 28s
Test Flutter beta on macos-latest	35m 1s
Test Flutter beta on windows-latest	23m 21s
Test Flutter master on ubuntu-latest	16m 20s
Test Flutter master on macos-latest	31m 26s
Test Flutter master on windows-latest	24m 17s
	4h 21m 52s

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