[build] Use GitInfo to generate $(Version) (#131)

Context: http://github.com/xamarin/xamarin-android/commit/2d81740cc1c708cd0474c9eac8936cd7abfc72e3
Context: dotnet/android#5749

In order to better support building solutions which reference both
.NET 6 and legacy projects within Visual Studio, we have begun
strong-naming all the MSBuild-related assemblies; see also 87acd6b.
This allows loading two different version of the "same" assembly, e.g.
`Microsoft.Android.Build.BaseTasks.dll`, from two different locations
within the same AppDomain/AssemblyLoadContext.

However, strong-naming is only part of the solution.  The other part
is that, for sanity and reliability, the "two different versions of
the 'same' assembly" should *also* have different assembly *versions*.
If they have the same assembly version, are they truly different?

Update the build system by adding a `Directory.Build.targets` and
`GitInfo.txt` file so that the the [`GitInfo`][0] [NuGet Package][1]
will be used to "fully" set the `$(Version)` MSBuild property,
consisting of the major and minor version numbers contained within
`GitInfo.txt`, along with a "computed" `$(GitSemVerPatch)` value
based on the "patch version" in `GitInfo.txt`, if present, added to
the number of commits which have occurred since `GitInfo.txt` changed.

Additionally, set the `$(InformationalVersion)` MSBuild property so
that it mirrors the behavior in [`Mono.Android.dll`][2]:

	[assembly: System.Reflection.AssemblyInformationalVersionAttribute("1.0.1; git-rev-head:4c2e36c; git-branch:main")

[0]: https://github.com/devlooped/GitInfo
[1]: https://www.nuget.org/packages/GitInfo/2.1.2
[2]: dotnet/android@b620689

Directory.Build.targets

@@ -0,0 +1,16 @@
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <ItemGroup>
+    <PackageReference Include="GitInfo" Version="2.1.2" PrivateAssets="all" />
+  </ItemGroup>
+  <PropertyGroup>
+    <GitThisAssembly>false</GitThisAssembly>
+    <GitDefaultBranch>main</GitDefaultBranch>
+  </PropertyGroup>
+
+  <Target Name="SetVersion" BeforeTargets="GetAssemblyVersion;GetPackageVersion" DependsOnTargets="GitVersion">
+    <PropertyGroup>
+      <Version>$(GitSemVerMajor).$(GitSemVerMinor).$(GitSemVerPatch)</Version>
+      <InformationalVersion>$(Version); git-rev-head:$(GitCommit); git-branch:$(GitBranch)</InformationalVersion>
+    </PropertyGroup>
+  </Target>
+</Project>

GitInfo.txt

@@ -0,0 +1 @@
+1.0