Allow Xamarin.Build.Download to optional bypass validate ID

[JimmyPun610]

Since invalid item id always occurs if the library is beta version, this update allow us bypass the validation so Xamarin.Build.Download will not block the build.

Issue related:
#1293
Invalid item ID image-1.0.0-beta1
Invalid item ID image-1.0.0-beta1

Accept property XamarinBuildDownloadBypassValidation from .csproj to bypass validation on downloading packages by adding these lines in csproj.

<Project>
....
	<Target Name="SetXamarinBuildDownloadBypassValidation" DependsOnTargets="BeforeBuild">
		<PropertyGroup>
			<XamarinBuildDownloadBypassValidation>true</XamarinBuildDownloadBypassValidation>
		</PropertyGroup>
	</Target>
....
</Project>

This will ignore error ErrorCodes.XbdInvalidItemId and download the beta package.

[net-foundation-cla]

All CLA requirements met.

[jpobst]

We expanded the RegEx to allow for prerelease id's so hopefully this isn't needed anymore:
#1384

I'm going to close this, but it can be re-opened in the future if we find a case that would be better solved with a "bypass" like this.