Fix CreateMessageRequestParams.Metadata type to JsonObject #2196

Job	Run time
build (windows-latest, Release)	11m 34s
build (ubuntu-latest, Debug)	6m 50s
build (macos-latest, Release)	4m 56s
build (ubuntu-latest, Release)	6m 58s
build (windows-latest, Debug)	11m 29s
build (macos-latest, Debug)	5m 47s
publish-coverage / publish-coverage	1m 5s
	48m 39s

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