feat: support PKCE

[tft7000]

Added support for PKCE (extracted from PR #86 and reduced to the minimum).

PKCE was implemented via the underlying library. For that I had to increase the minimum version for
league/oauth2-client to 2.7. I think this should be fine, as there seems not to be any dependency
differences between 2.6 and 2.7.

• Test added
• Documentation added; Note: README.md and CHANGELOG.md not updated
• Tests pass
• composer run validate passes

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (de04151) 71.08% compared to head (23edf4a) 72.35%.

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Additional details and impacted files

@@             Coverage Diff              @@
##             master      #87      +/-   ##
============================================
+ Coverage     71.08%   72.35%   +1.26%     
- Complexity      186      194       +8     
============================================
  Files            14       14              
  Lines           588      615      +27     
============================================
+ Hits            418      445      +27     
  Misses          170      170              

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[pradtke]

@tft7000 Thank you very much for this contribution.