Add MCP Apps extension support (typed metadata, attribute, and helpers) #2862

Job	Run time
build (ubuntu-latest, Release)	10m 59s
build (macos-latest, Debug)	11m 11s
build (macos-latest, Release)	9m 25s
build (windows-latest, Release)	17m 19s
build (ubuntu-latest, Debug)	11m 37s
build (windows-latest, Debug)	20m 30s
publish-coverage / publish-coverage	58s
	1h 21m 59s

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