Simple API with different data sources and different response types

[ FULLY FUNCTIONAL, BUT NOT MAINTAINED CODE (ONLY FOR DEMONSTRATION) ]

Assignment: Create production-ready ASP.NET Core service app that provides API for storage and retrive documents in different formats

1. The documents are send as a payload of POST request in JSON format and could be modified via PUT verb.
   3 API methods added: GET/POST/PUT for document operations

2. The service is able to return the stored documents in different format, such as XML, MessagePack, etc.
   The service is able to return documents in (by Accept header): JSON (application/json), XML (application/xml) and MessagePack (application/x-msgpack)

3. It must be easy to add support for new formats.
   New formats can easily be added to the services in the Startup.cs file (using the library or a custom formatter)

4. It must be easy to add different underlying storage, like cloud, HDD, InMemory, etc.
   It´s easy to switch between Memory and SQL Server storage in the appsettings.json file ("memory" or "sqlserver")

5. Assume that the load of this service will be very high (mostly reading).
   All operations are used as async methods

6. Demonstrate ability to write unit tests.
   Added 2 unit tests for cache memory operations, also added 3 integration tests to test all response formats JSON/XML/MessagePack

Note: If you want to run tests in SQL server mode, first run the application with the correct DB connection string (the database and table will be created automatically)