[feature] Add project(s) to a solution directory

[peterbomers]

The post action Add project(s) to a solution file with id D396686C-DE0E-4DE6-906D-291CD29FC5DE allows us to add projects to an existing solution.

It's however not possible to add a project to a solution folder inside the existing solution.
The dotnet cmd used by the template engine supports this by adding the --solution-folder argument.

It would be nice if could specify it like this:

  "postActions": [
    {
      "description": "Add projects to solution",
      "manualInstructions": [],
      "primaryOutputIndexes": "0",
      "solutionDirectory":  "unit-tests",
      "actionId": "D396686C-DE0E-4DE6-906D-291CD29FC5DE",
      "continueOnError": true
    }
  ]

[vlada-shubina]

The feature is implemented, JSON schema and documentation is updated