reflect/protoreflect: add helper methods to FieldDescriptor

Added API:
	FieldDescriptor.IsExtension
	FieldDescriptor.IsList
	FieldDescriptor.MapKey
	FieldDescriptor.MapValue
	FieldDescriptor.ContainingOneof
	FieldDescriptor.ContainingMessage

Deprecated API (to be removed in subsequent CL):
	FieldDescriptor.Oneof
	FieldDescriptor.Extendee

These methods help cleanup several common usage patterns.

Change-Id: I9a3ffabc2edb2173c536509b22f330f98bba7cf3
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/176977
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/legacy/file_test.go b/internal/legacy/file_test.go
index 9a9a3fc..c8d9af5 100644
--- a/internal/legacy/file_test.go
+++ b/internal/legacy/file_test.go
@@ -433,7 +433,7 @@
 					case "HasJSONName":
 						// Ignore this since the semantics of the field has
 						// changed across protoc and protoc-gen-go releases.
-					case "Oneof", "Extendee", "Enum", "Message":
+					case "ContainingOneof", "ContainingMessage", "Enum", "Message":
 						// Avoid descending into a dependency to avoid a cycle.
 						// Just record the full name if available.
 						//
@@ -442,6 +442,8 @@
 						if !v.IsNil() {
 							out[name] = v.Interface().(pref.Descriptor).FullName()
 						}
+					case "Oneof", "Extendee":
+						// TODO: Remove this.
 					default:
 						out[name] = m.Call(nil)[0].Interface()
 					}