RFC (core): Update Incremental Delivery response payload support

[kitten]

This RFC supersedes #2995.

Summary

See spec edits for incremental delivery support in graphql-spec: https://github.com/graphql/graphql-spec/pull/742/files#diff-98d0cd153b72b63c417ad4238e8cc0d3385691ccbde7f7674bc0d2a718b896ec
See updated implementation of graphql-js: graphql/graphql-js#3659

Previously, our current definition of ExecutionResult looks like the following:

urql/packages/core/src/types.ts

Lines 21 to 37 in 9ecfba4

	export type ExecutionResult =
	| {
	errors?:
	| Array<Partial<GraphQLError> | string | Error>
	| readonly GraphQLError[];
	data?: null | Record<string, any>;
	extensions?: Record<string, any>;
	hasNext?: boolean;
	}
	| {
	errors?:
	| Array<Partial<GraphQLError> | string | Error>
	| readonly GraphQLError[];
	data: any;
	path: (string | number)[];
	hasNext?: boolean;
	};

export type ExecutionResult =
  | {
      errors?:
        | Array<Partial<GraphQLError> | string | Error>
        | readonly GraphQLError[];
      data?: null | Record<string, any>;
      extensions?: Record<string, any>;
      hasNext?: boolean;
    }
  | {
      errors?:
        | Array<Partial<GraphQLError> | string | Error>
        | readonly GraphQLError[];
      data: any;
      path: (string | number)[];
      hasNext?: boolean;
    };

The spec has been updated to instead look like the following for incremental responses:

interface BaseExecutionResult {
  errors?:
    | Array<Partial<GraphQLError> | string | Error>
    | readonly GraphQLError[];
  extensions?: Record<string, any>;
}

interface BaseIncrementalDeliveryResult {
  label?: string;
  path: (string | number)[];
}

interface DeferResult extends BaseIncrementalDeliveryResult {
  data: unknown | null
}

interface StreamResult extends BaseIncrementalDeliveryResult {
  items: unknown[] | null
}

type IncrementalResult = DeferResult | StreamResult;

export interface ExecutionResult extends BaseExecutionResult {
  incremental?: IncrementalDeliveryResult[];
  data?: null | Record<string, any>;
  hasNext?: boolean;
}

Proposed Solution

• Update @urql/core with above types (with modifications?)
  • We probably don't want to support the old format, since it was experimental. Instead, I'd flag this as a minor with a note in the changelog that reflects why we've done this.
• Update mergeResultPatch to reflect the new format:

  urql/packages/core/src/utils/result.ts

  Lines 28 to 32 in 9ecfba4

  	export const mergeResultPatch = (
  	prevResult: OperationResult,
  	patch: ExecutionResult,
  	response?: any
  	): OperationResult => {

Requirements

• We must be able to comply with the new format in full; again, label is treated as optional and we don't currently use its information
• We should look into how extensions and errors are now treated, since there could be multiple ones sent per response.
• We should check what happens to result.extensions and result.errors if those can also be present on result.incremental[].extensions and result.incremental[].errors now