Insert Implicit PackageReference for Microsoft.SBOM.Targets into the .NET SDK

[baronfel]

Is your feature request related to a problem? Please describe.

When a user has <GenerateSBOM>true</GenerateSBOM> then the SDK should insert an up to date implicit PackageReference to the Microsoft.SBOM.Targets package, if the user has not already added one. This means we need to.

• have a tracking property for the version of Microsoft.SBOM.Targets to insert
• have automation to bump our tracking version of the package
• allow the user to override that property
• have a condition that checks the existing PackageReferences for this package
• have notification messages that tell the user the SDK can provide this reference

This work has already begun in #43151