Add Links to Pdf Generation

[ochsnerd]

Allows adding links while generating a Pdf.

The PdfDocumentBuilder already supports writing Link-Annotation, but the PdfPageBuilder did not yet have the appropriate Methods.

I created LinkAnnotation to make the specification of a link more ergonomic. The LinkAnnotation does not inherit from Annotation since supporting the whole breadth of Pdf-Annotations while writing the Pdf was too much work. Additionally, the design of the Annotation-class is more suited to parsing and does not work well when writing a Pdf.