feat: support useReadonlyTypes option

[jonaskello]

Fixes #163

This PR will add the option useReadonlyTypes. When set to true this option will generate code like this:

export interface Entity {
  readonly intVal: number;
  readonly stringVal: string;
  readonly intArray: readonly number[];
  readonly stringArray: readonly string[];
  readonly subEntity: SubEntity | undefined;
  readonly subEntityArray: readonly SubEntity[];
  readonly optionalIntVal?: number | undefined;
  readonly fieldMask: readonly string[] | undefined;
  readonly listValue: ReadonlyArray<any> | undefined;
  readonly structValue: { readonly [key: string]: any } | undefined;
}

[jonaskello]

@stephenh I went with the as any approach as discussed in #163, do you think that is OK?

[stephenh]

@jonaskello this looks great! Thanks for the PR, and also for testing the various google.protobuf.* well known types.

[stephenh]

🎉 This PR is included in version 1.130.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release
• v1.130.0

Your semantic-release bot 📦🚀

[jonaskello]

@stephenh Thank you for the quick review and merge. Also, thanks for creating this awesome package in the first place. It really makes a huge difference for working with protobuf in typescript :-).