Convenience APIs to add item with metadata as attributes

[mishra14]

Currently, ProjectItemGroupElement.AddItem(string itemType, string include, IEnumerable<KeyValuePair<string, string>> metadata); adds the metadata as children to the item and there is no way to add metadata as attributes.

e.g. -

<PackageReference Include="NewtonSoft.Json">
  <Version>9.0.1</Version>
</PackageReference>

As opposed to -

<PackageReference Include = "Newtonsoft.Json" Version="9.0.1"/>

//cc: @rainersigwald @rrelyea

[rainersigwald]

It's possible to do this by explicitly adding metadata but not using the convenience APIs that package the metadata creation into the item creation.

This seems like a reasonable request.

[Sarabeth-Jaffe-Microsoft]

@cdmihai Convenience APIs for adding metadata as attributes.