Extract check for unique operation types into separate rule

[vhenzl]

Introduces UniqueOperationTypes validation rule.

Corresponding change in the reference implementation: graphql/graphql-js@9b7a8af

[codecov]

Codecov Report

Merging #995 (e03e5e6) into master (816ae2a) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #995      +/-   ##
============================================
+ Coverage     94.47%   94.49%   +0.02%     
  Complexity       50       50              
============================================
  Files           119      120       +1     
  Lines          9617     9635      +18     
============================================
+ Hits           9086     9105      +19     
+ Misses          531      530       -1     

Impacted Files	Coverage Δ
src/Utils/BuildSchema.php	100.00% <ø> (ø)
src/Utils/SchemaExtender.php	99.33% <100.00%> (+0.31%)	⬆️
src/Validator/DocumentValidator.php	83.47% <100.00%> (+0.14%)	⬆️
src/Validator/Rules/UniqueOperationTypes.php	100.00% <100.00%> (ø)

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[spawnia]

Looks solid overall, can be micro-optimized a bit

[spawnia]

Can you add a CHANGELOG entry to mention the introduction of a new rule?

[vhenzl]

@spawnia It's ready.