Allow Targets metadata on projects in MSBuild task

The [`MSBuild` task](https://github.com/Microsoft/msbuild/blob/dd60c15e008f34a855024265743b76036ddb2dd2/src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/MSBuild.cs) allows building projects with different sets of properties simultaneously by setting the `AdditionalProperties` metadatum on the `Projects` input parameter, but to build specific targets per project, you must batch over the task and set different values for the `Targets` parameter.

It would be nice if the task paid attention to the `Targets` metadata so a project could build many projects, each with a unique list of targets, in parallel.

I noticed this when debugging a serialized portion of a build that came down to the VSSDK needing to build specific targets in each referenced project (to collect outputs from a user-defined list of targets) and being unable to fully parallelize that work.