Allow setting additional metadata on a project reference when switching to projects

[binginsin]

Working on an analyzer / code generator nuget I realised the only way to get it to work as a project reference was by setting additional attributes of

OutputItemType=Analyzer,
ReferenceOutputAssembly=true

But there is currently no way to achieve this behavior using switcher. This PR adds the functionality. An example switcher.json:

{
  "solution": "TestSolution.slnx",
  "solutionFolder": null,
  "mappings": {
    "TestNuget1": "../Common/TestNuget1.csproj",
    "CodeGeneratorNuget": "../Common/CodeGeneratorNuget.csproj"
  },
  "removeProjects": true,
  "additionalMetadata": {
    "CodeGeneratorNuget": [
      "OutputItemType=Analyzer",
      "ReferenceOutputAssembly=true"
    ]
  }
}

This will produce project references of:

<ProjectReference Include="../Common/TestNuget1.csproj" />
<ProjectReference Include="../Common/CodeGeneratorNuget.csproj">
  <OutputItemType>Analyzer</OutputItemType>
  <ReferenceOutputAssembly>true</ReferenceOutputAssembly>
</ProjectReference>

[binginsin]

@RicoSuter this repo needs some love! There's a few tiny PRs that are waiting to be merged that would fix a fair few issues.