[API Compat] Add MSBuild task to run API Compat standalone

[safern]

We should add the MSBuild Task to the Microsoft.DotNet.Compatibility package.

I have a prototype here which includes targets and props, it also accepts c# source as an input for the left hand side of the comparison: https://github.com/safern/apicompat/tree/main/src/Microsoft.DotNet.ApiCompatibility.Tasks

cc: @ericst @terrajobst @Anipik

[ViktorHofer]

As mentioned in #23631, it would be great if the new ApiCompat task would support passing in implementation assemblies instead or in addition to implementation directories. The legacy ApiCompat tooling doesn't support that.

[safern]

We should prioritize this for .NET 7 so that dotnet/runtime can start its migration to the new API Compat once the rules gap vs the old tooling is filled.

cc: @joperezr