Add KeptByAttribute to validate an item was kept due to a specific dependency (dotnet...

Add KeptByAttribute to validate an item was kept due to a specific dependency (dotnet/linker#3096)

The tests we have in the linker try to ensure that something can only be kept for a certain reason, but it can be hard to ensure it's not being kept for another reason. For example, sometimes we use typeof(TestType) to mark a type, but that also makes it relevant to variant casting, which can cause parts of TestType to be kept for unintended reasons.

This PR creates the KeptByAttribute which accepts a fully qualified string in Cecil format to indicate depenency provider (the thing that is creating the dependency that marks the item with the attribute), as well as a string representing the DependencyKind to specify the "reason" there was a dependency between the two. It also can accept a System.Type, or a System.Type + name of the member to indicate the dependency provider.

Commit migrated from https://github.com/dotnet/linker/commit/dc5e60f5f2becf0b462d37ad918443126e80b49b