Improve setup capabilities to avoid caller-provided data files in common use cases

[mara004]

Currently, the only way to do offline setup is using caller-provided data files, which is inconvenient, particularly with ctypesgen integration.

See the ideas in pypdfium2-team/ctypesgen#17 (comment) for new setup options that might allow for offline setup with higher-level interface and internally-managed ctypesgen:

[...] taking a full pdfium version (to avoid web calls to resolve it from the build cipher), a path to custom pdfium headers (e.g. $PDFIUM_HEADERS), and adding new system-path: and bundle: strategies to $PDFIUM_PLATFORM.