External data plugin, support for JSON files

[mharwig]

The PR extends the External Data Plugin to support JSON files, analogous to added support for excel/csv.

Code for handling the json is heavily based on https://github.com/kazantsev033/SpecFlow.Contrib.JsonData by @kazantsev033, but support was added for nested arrays of json objects.

The new concept is demonstrated by the /Plugins/Reqnroll.ExternalData/Reqnroll.ExternalData.ReqnrollPlugin.IntegrationTest/Features/ExternalDataFromJson.feature file.

Attempted to get this merged into SpecFlow before, see https://github.com/SpecFlowOSS/SpecFlow/pull/2700.

Checklist:

• [x] I've added tests for my code. (most of the time mandatory)

• [x] I have added an entry to the changelog. (mandatory)

• [x] My change requires a change to the documentation.

• [x] I have updated the documentation accordingly.

[gasparnagy]

@mharwig Super. I will review it in detail within a few days.