ASP.NET Core 6, Azure Cosmos DB .NET SDK v3 for SQL API, and General Refactoring

• Upgraded to ASP.NET Core 6.0.0
• Refactored to use Azure Cosmos DB .NET SDK v3 for SQL API
• Dropped HeadMethodMiddleware as it no longer works with endpoint routing
• Refactored conditional requests validators to utilize _etag and _ts from Azure Cosmos DB
• Introduced usage of IAsyncEnumerable
• Refactored from "by layer" organization to more "by feature" organization