Let workspace private and personal by default for all user

[hghandri]

Issue: https://github.com/aws-samples/aws-genai-llm-chatbot/issues/227

Implement private workspace, every user create a workspace now is only private and not shareable with other user. This feature implement a new DynamoBD table handle policies on top of Workspace Dynamodb table. This implementation could facilitate sharing workspaces between users with writeable / readable policy option through sharing process like Google Drive

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