Add ForEachPresentOption method to optionExtensionDescriptor (#2758)

Pulling changes to `protosource` from
#2752 into its own PR.

private/pkg/protosource/option_extension_descriptor.go

@@ -126,3 +126,10 @@ func isDescendantPath(descendant, ancestor []int32) bool {
 	}
 	return true
 }
+
+func (o *optionExtensionDescriptor) ForEachPresentOption(fn func(protoreflect.FieldDescriptor, protoreflect.Value) bool) {
+	// Note: This does not bother to handle unrecognized fields.
+	// Should not be a problem since descriptors models in the buf CLI codebase should have them
+	// all correctly parsed and known.
+	o.message.ProtoReflect().Range(fn)
+}

private/pkg/protosource/protosource.go

@@ -148,9 +148,16 @@ type OptionExtensionDescriptor interface {
 	// If no relevant location is found in source code info, this returns nil.
 	OptionExtensionLocation(extensionType protoreflect.ExtensionType, extraPath ...int32) Location
 
-	// PresentExtensionNumbers returns field numbers for all options that
-	// have a set value on this descriptor.
+	// PresentExtensionNumbers returns field numbers for all extensions/custom options
+	// that have a set value on this descriptor.
 	PresentExtensionNumbers() []int32
+
+	// ForEachPresentOption iterates through all options that have a set value on this
+	// descriptor, invoking fn for each present option.
+	//
+	// If fn returns false, the iteration is terminated and ForEachPresentOption
+	// immediately returns.
+	ForEachPresentOption(fn func(protoreflect.FieldDescriptor, protoreflect.Value) bool)
 }
 
 // Location defines source code info location information.