Add encryption & protection flags support

[AndreRenaud]

There is a reasonable description of this available at https://www.cs.cmu.edu/~dst/Adobe/Gallery/anon21jul01-pdf-encryption.txt and https://www.cs.cmu.edu/~dst/Adobe/Gallery/
Some further details are available at https://github.com/pdfcpu/pdfcpu

[AndreRenaud]

There is a sample encrypted file available from https://johnaugust.com/2014/try-to-open-this-pdf
And a discussion of opening/cracking it (including the code) at https://blog.didierstevens.com/2017/12/26/cracking-encrypted-pdfs-part-1/