Update XMP metadata directly

[stefan6419846]

Explanation

The XmpInformation class for holding XMP metadata is currently restricted to read data. To actually update the data, the user has to mess with the associated XML through rdf_root. This may lead to strange results as well when first accessing a value and then updating it, as on the first read access, the values will be cached internally.

Code Example

from pypdf import PdfReader, PdfWriter

writer = PdfWriter(clone_from="file.pdf")
metadata = writer.xmp_metadata
assert metadata is not None
print(metadata.dc_author)
metadata.dc_author = "Foo Bar"
print(metadata.dc_author)
writer.write("output.pdf")