feat: Manipulate layers / optional content

[p-vdp]

Please explain your intent
The ability to turn layers on/off, delete layers, and copy layers to a new file

Describe the solution you'd like
A new command layers, perhaps something like:

> pdfly layers doc.pdf --list
['layer1': {'visible': True}, 'layer2': {'visible': True}]
> pdfly layers doc.pdf --off layer1
layer1 turned off
> pdfly layers doc.pdf --list
['layer1': {'visible': False}, 'layer2': {'visible': True}]
> pdfly layers doc.pdf --del layer2
layer2 deleted
> pdfly layers doc.pdf --list
['layer1': {'visible': False}]

Additional context
I'm interested in working on this!

[p-vdp]

After a little research I see this is a longstanding upstream issue in pypdf: py-pdf/pypdf#1181

I'll take this up over there for now.

[Lucas-C]

This would be a nice addition to pdfly 👍