160 add overlapping fields can be merged validation rule (graphql-dotnet#1005)

* first try

* update OverlappingFieldsCanBeMerged rule

* add test cases for OverlappingFieldsCanBeMerged rule

* add more test cases

* fix Conflicting args should fail test case

* Fix test Encounters_conflict_in_fragments_should_fail

* Add more tests and fix the logic

* Add toString override to print the GraphType

* make schema configurable in ValidationTest Config

* Add more test cases and fixed the logic

* change rule error code from 5.3.2.2 to 5.3.2

* update readme.md

Author: KoditkarVedant

README.md

@@ -181,7 +181,7 @@ var json = schema.Execute(_ =>
 - [x] No undefined variables
 - [x] No unused fragments
 - [x] No unused variables
-- [ ] Overlapping fields can be merged ([help with this rule](https://github.com/graphql-dotnet/graphql-dotnet/issues/160))
+- [x] Overlapping fields can be merged
 - [x] Possible fragment spreads
 - [x] Provide non-null arguments
 - [x] Scalar leafs