Support loading plugin dependencies from .deps.json on .NET Framework and Visual Studio MSBuild

[obligaron]

🤔 What's changed?

• When loading plugins, we also look for .deps.json and use the content to load dependencies (of the plugin).
• Added documentation for PlugIn Writer
• Added regression tests for NUnit Retry (which uses .deps.json)

⚡️ What's your motivation?

Fixes #410 and standardises the behaviour of .NET 6+ and the .NET Framework.
NET 6+ already uses `DependencyContext', with this PR the .NET Framework does as well.

🏷️ What kind of change is this?

• 📖 Documentation (improvements without changing code)
• 🐛 Bug fix (non-breaking change which fixes a defect)

♻️ Anything particular you want feedback on?

We could also use this approach to reduce the size of Reqnroll.Tools.MsBuild.Generation. But that should be part of another PR.

📋 Checklist:

• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "[vNext]" section of the CHANGELOG, linking to this pull request & included my GitHub handle to the release contributors list.

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[gasparnagy]

Thx. I refactored the resolvers a bit, but this is fine.