Handle files >4GB in g_file_load_contents()

Open Benjamin Otte requested to merge wip/otte/large-files into main

This MR implements support for files >4GB for g_file_load_contents() and in turn g_file_load_bytes() and their async variants.

The current code uses GByteArray and that uses guint as size, so the code causes a buffer overflow.

I'm not sure how to create testcases for this, because I don't think CI likes creating files >4GB?

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