feat(Cosmos): Use Azure.Cosmos STJ impl

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• Use the in-the-box System.Text.Json support provided in Microsoft.Azure.Cosmos
• CosmosStoreConnector: Remove requestTimeout mandatory argument - the particulars of tuning that are a lower level concern best left to the SDK team to manage (they've adjusted it from 5 to 6 seconds internally, and their retry logic is tuned and managed in accordance with the CosmosDB Service's SLAs and/or behavior under stress to a degree that neither Equinox or applications have much business concerning themselves with)
  • NOTE the parameter is still exposed for backcompat, and it continues to be something you want to consider taking ownership of if your calls/queries affect the user experience