utils: Add stash and dispose methods for readonly contents

[JasonYeMSFT]

• The dispose method will allow an extension that uses the ReadOnlyContentProvider to offer a manual way to clear the contents stashed in its provider. This is not ideal but I think it's still better than not allowing people to dispose them at all.
• The stash method adds the content to the internal content cache without opening the document. I want to use it in the Azure agent extension so I can present read-only content as a reference link in the chat window.