[mono] Add MonoAOTCompiler MSBuild Task to produce NuPkg (#46537)

[mono] Add MonoAOTCompiler MSBuild Task to produce NuPkg (#46537)

* Test workload

* Remove hardcoded path to MonoAOTCompiler

Remove changes related to testing the MonoAOTCompiler NuPkg

Remove changes to deprecated files that led to file additions

* Inline pkg descriptions for MonoAOTCompiler Task and workload packs

* Remove Workload AOT additions

* Replace _ILLinkTasksDirectoryRoot with _AOTTasksDirectoryRoot

* Add TargetsMobile condition

* Remove temporary variables

* Reword MonoAOTCompiler NuPkg description

* Remove NuPkg import of MonoAOTCompiler UsingTask

* Add MonoAOTCompilerTaskAssemblyPath property

* Cleanup Microsoft.NET.Runtime.MonoAOTCompiler.Tasks.props

* Add README.md with example of using the NuPkg

* Specify the package files for the aot task, in the project itself

* Add comment for GetFilesToPackage target

* Update README.md

Co-authored-by: Ankit Jain <radical@gmail.com>
Co-authored-by: Steve Pfister <steve.pfister@microsoft.com>
Co-authored-by: Mitchell Hwang <mitchell.hwang@microsoft.com>
Co-authored-by: Ankit Jain <radical@gmail.com>