Polish pg_sheet_fdw

[Hyrikan]

• investigate memory allocation (is it possible to know exactly how much memory we will need for the whole table in advance?)
• investigate postgres tuple polling speed and sheetreader polling speed. Maybe having a simple threaded pipeline already is a big advantage.
• investigate postgres multithreaded fdw interface

[Hyrikan]

added virtual tuples.

[Hyrikan]

reduced string copying to a feasible minimum by accessing the memory of the cpp XlsxCell type like it is a similar spaced c type. This makes it necessary to call the readout of the dynamic xlsx string from c directly. We arrive at one .c_str() call in cpp and two copies of the string, from which one is mandatory in pg!

For the future, the copie could be further reduced to one by accessing the mSharedStrings array in the XlsxFile directly from c and ignoring the interaction methods of the XlsxFile.