Allow custom cell_fill_mode in tables

[Lucas-C]

It will now be possible to define your own cell-filling logic:

class EvenOddCellFillMode():
    @staticmethod
    def should_fill_cell(i, j):
        return i % 2 and j % 2

with pdf.table(cell_fill_color=lightblue, cell_fill_mode=EvenOddCellFillMode()) as table:
    ...

[Lucas-C]

I added two new enum values EVEN_ROWS & EVEN_COLUMNS:

• test code: https://github.com/py-pdf/fpdf2/blob/allow-custom-cell-fill-mode/test/table/test_table.py#L760
• resulting PDF: https://github.com/py-pdf/fpdf2/blob/allow-custom-cell-fill-mode/test/table/table_cell_fill_mode.pdf

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 93.27%. Comparing base (b7e9b11) to head (218a0af).

❗ Current head 218a0af differs from pull request most recent head fc56c54. Consider uploading reports for the commit fc56c54 to get more accurate results

Files	Patch %	Lines
fpdf/enums.py	93.75%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1117   +/-   ##
=======================================
  Coverage   93.26%   93.27%           
=======================================
  Files          30       30           
  Lines        9215     9228   +13     
  Branches     2099     2103    +4     
=======================================
+ Hits         8594     8607   +13     
  Misses        379      379           
  Partials      242      242           

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