Extract method to get TCPDF producer

[MauricioFauth]

Wraps TCPDF_STATIC::getTCPDFProducer() calls with a private method and adds an empty private property that can be used to change the producer line. The private property can be changed with the reflection API.

Both method and property are declared as private to avoid BC breaks.

The idea is to help with testing PDF generation, by using a predictable producer line.

• Related to https://github.com/tecnickcom/TCPDF/pull/85

[codecov-commenter]

Codecov Report

Merging #572 (d936d51) into main (2fb1c01) will decrease coverage by 0.01%. The diff coverage is 57.14%.

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@@            Coverage Diff             @@
##             main     #572      +/-   ##
==========================================
- Coverage   69.33%   69.32%   -0.01%     
==========================================
  Files         128      128              
  Lines       27022    27027       +5     
==========================================
+ Hits        18735    18737       +2     
- Misses       8287     8290       +3     

Flag	Coverage Δ
php-5.3-ubuntu-latest	?
php-5.4-ubuntu-latest	68.84% <57.14%> (-0.01%)	:arrow_down:
php-5.5-ubuntu-latest	?
php-5.6-ubuntu-latest	68.91% <57.14%> (-0.01%)	:arrow_down:
php-7.0-ubuntu-latest	71.58% <66.66%> (-0.01%)	:arrow_down:
php-7.1-ubuntu-latest	73.06% <80.00%> (-0.01%)	:arrow_down:
php-7.2-ubuntu-latest	72.94% <80.00%> (-0.01%)	:arrow_down:
php-7.3-ubuntu-latest	?
php-7.4-ubuntu-latest	?
php-8.0-ubuntu-latest	?
php-8.1-ubuntu-latest	72.94% <80.00%> (-0.01%)	:arrow_down:
php-8.2-ubuntu-latest	?
php-nightly-ubuntu-latest	?

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Impacted Files	Coverage Δ
tcpdf.php	67.89% <57.14%> (-0.01%)	:arrow_down:

... and 1 file with indirect coverage changes

[williamdes]

@nicolaasuni could you review this one please ?

[nicolaasuni]

The producer line is intentionally hardcoded. New features like this one could be considered for the new tc-lib-pdf project.