feat: add support for field resolvers (#1323)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Field resolver support expanded: Product (shippingEstimate,
recommendedCategory) and Category (productCount, popularityScore,
categoryMetrics, subcategories).
* New schema types and inputs: Subcategory, CategoryMetrics,
ProductCountFilter, SubcategoryItemFilter, ShippingEstimateInput,
ShippingDestination enum, and openfed__FieldSet scalar.

* **Improvements**
* Execution engine now builds and runs dependency-aware, parallel RPC
call plans for resolver-driven and federated flows, with richer response
merging.

* **Tests & Tooling**
* New tests, benchmarks, mock service handlers, and mapping generation
helpers; package Readme added.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

## Checklist

- [ ] I have discussed my proposed changes in an issue and have received
approval to proceed.
- [ ] I have followed the coding standards of the project.
- [ ] Tests or benchmarks have been added or updated.

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Please add any additional information or context regarding your changes
here.
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Author: Noroth

v2/pkg/ast/ast.go

@@ -236,6 +236,11 @@ func (d *Document) NodeByNameStr(name string) (Node, bool) {
 	return d.Index.FirstNodeByNameStr(name)
 }
 
+// ResolveNodeFromTypeRef returns the `ast.Node` for a given type reference.
+func (d *Document) ResolveNodeFromTypeRef(typeRef int) (Node, bool) {
+	return d.Index.FirstNodeByNameStr(d.ResolveTypeNameString(typeRef))
+}
+
 func (d *Document) TypeDefinitionContainsImplementsInterface(typeName, interfaceName ByteSlice) bool {
 	typeDefinition, exists := d.Index.FirstNodeByNameBytes(typeName)
 	if !exists {

v2/pkg/ast/ast_selection.go

@@ -3,6 +3,7 @@ package ast
 import (
 	"bytes"
 	"fmt"
+	"strings"
 
 	"github.com/wundergraph/graphql-go-tools/v2/pkg/internal/unsafebytes"
 	"github.com/wundergraph/graphql-go-tools/v2/pkg/lexer/position"
@@ -241,3 +242,18 @@ func (d *Document) SelectionSetFieldNames(set int) (fieldNames []string) {
 	}
 	return
 }
+
+// SelectionSetFieldSetString returns a string of the field names in the selection set separated by a space
+// Example: "{ name status }" -> "name status"
+func (d *Document) SelectionSetFieldSetString(set int) string {
+	fieldSelections := d.SelectionSetFieldSelections(set)
+	builder := strings.Builder{}
+	for i, fieldSelection := range fieldSelections {
+		builder.Write(d.FieldNameBytes(d.Selections[fieldSelection].Ref))
+		if i != len(fieldSelections)-1 {
+			builder.WriteRune(' ')
+		}
+	}
+
+	return builder.String()
+}

v2/pkg/ast/path.go

@@ -33,6 +33,10 @@ type PathItem struct {
 
 type Path []PathItem
 
+func (p Path) Len() int {
+	return len(p)
+}
+
 func (p Path) Equals(another Path) bool {
 	if len(p) != len(another) {
 		return false
@@ -77,6 +81,28 @@ func (p Path) WithoutInlineFragmentNames() Path {
 	return out
 }
 
+func (p Path) WithPathItem(element PathItem) Path {
+	res := make(Path, len(p)+1)
+	copy(res, p)
+	res[len(res)-1] = element
+	return res
+}
+
+func (p Path) WithFieldNameItem(fieldName []byte) Path {
+	return p.WithPathItem(PathItem{
+		Kind:      FieldName,
+		FieldName: fieldName,
+	})
+}
+
+func (p Path) RemoveLastItem() Path {
+	if len(p) == 0 {
+		return p
+	}
+
+	return p[:len(p)-1]
+}
+
 func (p Path) String() string {
 	out := "["
 	for i := range p {