Allow extending existing fields

[egoodwinx]

Summary

This PR adds the ability to extend and modify existing fields when using GraphQL schema extensions, enabling more flexible schema composition patterns. Previously, extending a type with a field that already existed would cause a validation error. Now, extensions can override field properties and additively merge field arguments.

Motivation

RFC: graphql/graphql-spec#1162 (comment)

Changes

Core Implementation
extendSchema.ts

Added mergeFieldConfigs() function to intelligently merge field configurations
Added mergeFieldMaps() function to merge field maps from extensions
Modified buildFieldMap() to detect and merge duplicate field definitions
Updated extendObjectType() and extendInterfaceType() to use field merging
Merging Behavior:

Field type: Incoming type overrides existing (allows type evolution)
Description: Incoming description overrides existing
Arguments: Merged additively (existing + new arguments)
Deprecation reason: Incoming reason overrides existing

Tests

src/utilities/tests/extendSchema-test.ts

Added comprehensive test suite for field merging scenarios:
✅ Merging field types when extending with same field name
✅ Merging field arguments additively
✅ Merging multiple extensions on the same field
✅ Merging field descriptions
✅ Preserving original field properties when not overridden
✅ Allowing overriding field properties with extensions
✅ Interface field merging
✅ Complex multi-extension scenarios

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[egoodwinx]

@martinbonnin 👀