feat(Cosmos): Use Azure.Cosmos STJ impl

[bartelink]

NOTE 4.1.0-alpha.15+16 is based on this branch - if anyone wants/needs this, say the word and we can release a 4.0.5

• 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