added basic support for Free Text annotations

[MarekT0v]

#949
Currently it supports basic Free Text annotations.
"Basic"> Meaning, that one can't set annotation's color, font or font size. These are set by default now to black color, Helvetica, 12.

Checklist:

• The GitHub pipeline is OK (green),
  meaning that both pylint (static code analyzer) and black (code formatter) are happy with the changes of this PR.

• A unit test is covering the code added / modified by this PR

• This PR is ready to be merged

• In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder

• A mention of the change is present in CHANGELOG.md

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (bc44876) 93.58% compared to head (71a009f) 93.57%.
Report is 2 commits behind head on master.

❗ Current head 71a009f differs from pull request most recent head 3e5c15c. Consider uploading reports for the commit 3e5c15c to get more accurate results

Files	Patch %	Lines
fpdf/fpdf.py	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1039      +/-   ##
==========================================
- Coverage   93.58%   93.57%   -0.02%     
==========================================
  Files          29       29              
  Lines        8514     8530      +16     
  Branches     1566     1572       +6     
==========================================
+ Hits         7968     7982      +14     
- Misses        338      339       +1     
- Partials      208      209       +1     

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[Lucas-C]

This is promising!
Good job @MarekT0v 👍

black & pylint both validate your PR 😊

Have you checked that those Free Text annotations are visible when using a PDF reader?
If so, could share a screenshot please?

What remains to be done in order for this PR to be complete and that we merge it:

• at least one unit test covering the code added, maybe added to test/test_annotations.py.
  We have dome documentation on how to do this: https://py-pdf.github.io/fpdf2/Development.html#assert_pdf_equal-writing-new-tests
  By adding a reference PDF file this way, our PDF checkers will be able to validate it.
• some documentation in the docs/Annotations.md
• a mention of this addition in CHANGELOG.md

[MarekT0v]

Thank you @andersonhc, your review helped me a lot. I implemented the changes in the way as you suggested. Color, font and size can be now set for free text annotations.

[Lucas-C]

I made a single review comment.
Apart from that, I think this PR is ready to be merged.

@allcontributors please add @MarekT0v for code

[allcontributors]

@Lucas-C

I've put up a pull request to add @MarekT0v! 🎉

[Lucas-C]

Merged!

Thank you very much for your contribution @MarekT0v 👍