✨ basic PDF support

[aaronleopold]

Resolves #35

With these changes, PDFs are mostly supported:

• PDFs can be scanned, and metadata (if any) can be extracted
• PDFs can have generated thumbnails
• PDF pages can be streamed
• PDF files can be sent to the client for download and/or viewed in a native PDF viewer

This pulls in two additional libraries in the core:

1. pdf
2. pdfium-render

The pdfium-render allows the core to properly process PDF pages as images. However it should be noted the concerns brought up from #156 (comment). I think, considering how few options there are, this will just have to be tried and tested over time.

There are some areas that need improvements, but I will follow up with them at another time:

• PDF page streaming will only output PNG images for the time being. This is hard coded, but will be replaced with some sort of option to configure it
• I added basic PDF-to-ZIP conversion support in the core, but am not hooking it up with UI elements yet. I'll leave that for the dedicated ticket for that feature.

[aaronleopold]

A little update after some experimentation with the Docker builds locally. The added dependencies (the PDF crate and then the PDFium binary) and the additional PDFium stage to the docker build seem to have added about 40MB to the uncompressed size, totaling about 160MB now 😭 This isn't awful, but adds up when factored together with the switch from Alpine to Debian so I'd like to try and maintain and not add too much more. I imagine there are some fancier things to be done with the layers that can reduce this increase, but it's fine for now. Otherwise, everything functions great!