Add the ability to use schema providers

[msmakouz]

• Waiting for release: https://github.com/cycle/schema-provider
• Add unit tests
• Write docs about schema providers

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (e0bda2c) 76.82% compared to head (260ec0c) 80.91%.

Files	Patch %	Lines
src/Schema/Provider/AnnotatedSchemaProvider.php	65.71%	12 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x      #91      +/-   ##
============================================
+ Coverage     76.82%   80.91%   +4.08%     
+ Complexity      428      406      -22     
============================================
  Files            50       49       -1     
  Lines          1273     1226      -47     
============================================
+ Hits            978      992      +14     
+ Misses          295      234      -61     

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