Add ability to start with an existing PDF as the base

[beauwest]

We have encountered several situations where we are simply filling in values on a pre-existing form. In those cases we have had to manually export the original form PDF as an image, and then include the image as a background image on the report.

It would be very nice, and result in smaller report sizes, and faster generation, to be able to create a new report with an existing PDF as the base, and then use FluentReports to add text to it.

[NathanaelA]

@beaudesigns - This is a awesome idea. I know we have a way to import a pdf and use it as existing pages. We could probably allow additional page updates to allow modification of a page.

However, faster generation would be just importing an image. Reading the existing PDF actually is a much heavier process than just embedding an image.