allow open reads on Messages so that syncs can sync other people's messages

[swyxio]

allow open reads on Messages, fixes https://github.com/aws-samples/aws-appsync-chat/issues/6

Issue #, if available: https://github.com/aws-samples/aws-appsync-chat/issues/6

Description of changes: specifies auth operations on Messages so that reads are not auth required.

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