V2: Use JsonObject to represent google.protobuf.Struct

[timostamm]

The well-known message google.protobuf.Struct stores a dynamic object with the same range of types as a plain object in JSON. Unfortunately, it is cumbersome to construct a Struct message, and we recommend to use fromJson() to do so.

This PR changes the generated code to represent google.protobuf.Struct with JsonObject. For example:

syntax = "proto3";
import "google/protobuf/struct.proto";

message Example {
  google.protobuf.Struct struct = 1;
}

Generated code before:

import { Struct } from "@bufbuild/protobuf/wkt";

export type Example = Message<"Example"> & {
  /**
   * @generated from field: google.protobuf.Struct struct = 1;
   */
  struct?: Struct;
 
}

Generated code with this PR:

import { JsonObject } from "@bufbuild/protobuf";

export type Example = Message<"Example"> & {
  /**
   * @generated from field: google.protobuf.Struct struct = 1;
   */
  struct?: JsonObject;
}

JsonObject is defined as:

type JsonObject = { [k: string]: JsonValue };
type JsonValue =
  | number
  | string
  | boolean
  | null
  | JsonObject
  | JsonValue[];

The change applies to all fields, singular and repeated, map values, oneof members, and extensions. The generated messages Struct, Value, and ListValue from @bufbuild/protobuf/wkt are exempt, and remain unchanged.

Bundle size increases by 2%, performance does not regress.