Demonstrate how to choose DbContext per request

[bkoelman]

This PR answers the question at #1532.
Note this only works properly if the EF Core models (not the database tables) are identical.

Output:

GET http://localhost:14141/api/people?include=books

{
  "links": {
    "self": "/api/people?include=books"
  },
  "errors": [
    {
      "id": "f2b81ea6-dac4-48cc-a1f1-f5eb402eb140",
      "status": "400",
      "title": "The 'dbType' query string parameter is missing or invalid.",
    }
  ],
  "included": []
}

GET http://localhost:14141/api/people?include=books&dbType=sqlite

GET http://localhost:14141/api/people?include=books&dbType=postgresql

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.79%. Comparing base (9c51fe3) to head (bcaf5aa).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1535   +/-   ##
=======================================
  Coverage   90.79%   90.79%           
=======================================
  Files         346      346           
  Lines       11063    11063           
  Branches     1814     1814           
=======================================
  Hits        10045    10045           
  Misses        669      669           
  Partials      349      349           

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