ENH: Add decryption to the cat operation

[EmperorArthur]

I recently needed to decrypt a PDF file, and found the program would just error. This resolves that issue.

• Ads an optional password parameter to the "cat" operation. Allowing decryption.
• Makes decryption errors more readable for end-users.

[ouqilbo]

Love this. Started using pdfly a week ago and I was about to stop because I had to manipulate a password protected pdf

[Lucas-C]

Hi
Thank you for your contribution @EmperorArthur 👍
I think this PR needs to be rebased on main.
And pdfly/cat.py needs to be pretty-formatted with black

[ouqilbo]

Hey @EmperorArthur, could you please proceed with the pr and follow the previous suggestions? As I already said, this is really beneficial when in need to use password protected pdfs.

[MartinThoma]

Thank you for your contribution @EmperorArthur 🙏

[MartinThoma]

@ouqilbo This will be part of the next release. You can always install the current development version like this: https://pdfly.readthedocs.io/en/latest/user/installation.html#development-version