Make GraphQL schema overridable

New Feature / Enhancement Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.

Current Limitation

When trying to tighten the GraphQL type definitions by example changing a Parse number field to be Int instead of Float in the GraphQL schema.
Currently getting this error in the server logs, when trying to override a number field with type Int:

error: Error: Field "MyClass.numberField" already exists in the schema. It cannot also be defined in this type extension.
  at assertValidSDLExtension (/parse-server/node_modules/graphql/validation/validate.js:124:11)
  at Object.extendSchema (/parse-server/node_modules/graphql/utilities/extendSchema.js:77:43)
  at /parse-server/node_modules/@graphql-tools/stitch/index.cjs.js:1232:28
  at Array.forEach (<anonymous>)
  at stitchSchemas (/parse-server/node_modules/@graphql-tools/stitch/index.cjs.js:1231:16)
  at ParseGraphQLSchema.load (/parse-server/lib/GraphQL/ParseGraphQLSchema.js:212:56)
  at processTicksAndRejections (internal/process/task_queues.js:93:5)
  at async ParseGraphQLServer._getGraphQLOptions (/parse-server/lib/GraphQL/ParseGraphQLServer.js:61:17)
  at async /parse-server/lib/GraphQL/ParseGraphQLServer.js:99:86

Feature / Enhancement Description

Allow overriding of existing field types in the GraphQL schema.

Example Use Case

1. Create a class, MyClass, with a field of type Number
2. Extend graphql schema by adding

extend type MyClass {
  numberField: Int
}

# If I also know that all my numbers are all integers, then I can do this too:
extend input NumberWhereInput {
  equalTo: Int
  notEqualTo: Int
  lessThan: Int
  lessThanOrEqualTo: Int
  greaterThan: Int
  greaterThanOrEqualTo: Int
  in: [Int]
  notIn: [Int]
}

Alternatives / Workarounds

Currently working with Float instead of Int but would prefer to have a clear distinction between actual Floats and Ints.

Start in lib/GraphQL/ParseGraphQLSchema.js around ParseGraphQLSchema.load, then reproduce the MyClass example with an existing Number field and an Int extension. Trace the schema stitching and extension validation shown in the error. Done means existing GraphQL field types can be overridden, including the NumberWhereInput fields, without the duplicate-field error.