Improve PyPDFToDocument

[anakin87]

Currently this component is not very flexible or easy to customize.

In the past, we tried to make this component more flexible in #6258, introducing the possibility to pass custom PyPDFConverters.
Then we had issues with serialization (#6388) and solved the problem by temporarily introducing a registry (#6396).

This makes it difficult for users to introduce custom PyPDFConverters (somewhat related: #6896).

As discussed with Silvano, now that proper type serialization is in place, we would like to

• get rid of the registry
• introduce to_dict/from_dict in the PyPDFConverter protocol
• incorporate these changes into the Default Converter, which can also be a good example