ENH: Add command to export annotations as JSON

[MartinThoma]

This was originally asked in https://github.com/py-pdf/pypdf/issues/2291

Proposed syntax:

$ pdfly annotations export
[
{"page_index": 0, "/Subtype": }
]

So essentially the idea is to export the same as defined in 12.5 Annotations (PDF 1.7 specifications), but add the "page_index" to the dictionary.

For the moment, we would only support the JSON export. If we want to add more export formats, we can add --format with a default value of JSON.