Add MSBuild support for controlling --verbosity, --configuration, --platform and project file arguments to slngen.exe

[jdrobison]

This change gives project authors greater control over the parameters passed to slngen.exe by the SlnGen task:

slngen.exe Parameter	MSBuild property/item	Default value
--verbosity	SlnGenVerbosity (property)	Normal
--configuration	SlnGenConfiguration (property)	Unspecified
--platform	SlnGenPlatform (property)	Unspecified
Projects in solution	SlnGenProjects (item)	The project being built

Here's an example of project authoring using the new MSBuild properties and items:

<PropertyGroup Label="SlnGen configuration properties">
  <SlnGenLaunchVisualStudio>false</SlnGenLaunchVisualStudio>
  <SlnGenSolutionFileFullPath>$(MSBuildProjectDirectory)\$(MSBuildProjectName).sln</SlnGenSolutionFileFullPath>
  <SlnGenFolders>true</SlnGenFolders>
  <SlnGenVerbosity>Minimal</SlnGenVerbosity>
  <SlnGenPlatform>AnyCPU</SlnGenPlatform>
  <SlnGenConfiguration>Debug;Release</SlnGenConfiguration>
</PropertyGroup>
<ItemGroup Label="SlnGen configuration items">
  <SlnGenProjects Include="**\*.csproj" />
  <SlnGenProjects Include="..\..\..\UnitTests\**\File*.csproj" />
</ItemGroup>

[jdrobison]

Does a completed PR automatically result in publishing an updated NuGet package, or is there a separate process for that? If the former, what is the typical latency between PR completion and the NuGet package update?

Also, what is the process for updating https://microsoft.github.io/slngen ?

@mruxmohan4, @AndyGerlicher

[mruxmohan4]

There is a separate process which requires approval for package updates; however, there should be minimal latency between PR completion and package updates (should be in effect same day as PR completion).

[jdrobison]

There is a separate process which requires approval for package updates; however, there should be minimal latency between PR completion and package updates (should be in effect same day as PR completion).

Ah, is that the process that's documented in RELEASE.md?

[mruxmohan4]

Yes, I will add a note that repo maintainers are responsible for those package pushes.
Just reviewed the issue and changes - MSBuild properties/items seem appropriate for this use case.
Thank you for your contribution.

Regarding https://microsoft.github.io/slngen/, I will create a separate PR to update documentation (along with the changes made here). Generally, those changes are made here: https://github.com/microsoft/slngen/tree/main/docs