A76xx: Rewrote https_post_file

[mathieucarbou]

@lewisxhe: https_post_file leads to a compiler warning. After close inspection, I am wondering if it is correctly written... There is no bound check and a comparison with a reference of a - maybe - character ?

So he is a proposed implementation which seems more correct to me and also fixes the warning.

In file included from src/TinyGsmClientA7670.h:30,
                 from src/TinyGsmClient.h:66,
                 from /Users/mat/Data/Workspace/forks/TinyGSM/tools/test_build/test_build.ino:7:
src/TinyGsmHttpsA76xx.h: In member function 'int TinyGsmHttpsA76xx<modemType>::https_post_file(const char*, uint8_t, bool)':
src/TinyGsmHttpsA76xx.h:291:26: warning: comparing the result of pointer addition '(filepath + 3)' and NULL [-Waddress]
  291 |         if (&filepath[3] == NULL)return -1;
      |                          ^

Same as other PR, the first commit includes #4 (CI) in order to get build feedback (which can be seen in my fork: https://github.com/mathieucarbou/lewisxhe-TinyGSM/actions).

The commit relevant to this PR is the second one.