refactor(governance-sdk): DRYer Borsh classes, remove repeated properties & constructors

[johnrees]

This is a proposal to refactor serializable Borsh classes so that there is no repetition of their arguments, as this can create a lot of extra lines of code that devs might potentially forget to update when adding/removing/updating struct properties.

Example 1. Class with no custom properties and/or instance methods

before

export class ChatMessageBody {
  type: ChatMessageBodyType;
  value: string;

  constructor(args: { type: ChatMessageBodyType; value: string }) {
    this.type = args.type;
    this.value = args.value;
  }
}

// dev needs to specify/assign `type` and `value` multiple times

after

export interface ChatMessageBody {
  // only need to declare `type` and `value` in one place
  type: ChatMessageBodyType;
  value: string;
}
// ChatMessageBody is empty because constructor initialization is handled by BorshClass
export class ChatMessageBody extends BorshClass<ChatMessageBody> {}

---

Example 2. Class with custom properties and/or instance methods

before

export class ChatMessage {
  accountType = GovernanceChatAccountType.ChatMessage;

  proposal: PublicKey;
  author: PublicKey;
  postedAt: BN;
  replyTo: PublicKey | undefined;
  body: ChatMessageBody;

  constructor(args: {
    proposal: PublicKey;
    author: PublicKey;
    postedAt: BN;
    replyTo: PublicKey | undefined;
    body: ChatMessageBody;
  }) {
    this.proposal = args.proposal;
    this.author = args.author;
    this.postedAt = args.postedAt;
    this.replyTo = args.replyTo;
    this.body = args.body;
  }
}

after

// intermediary interface declared as extra property `accountType` is not to be passed to the constructor
interface ChatMessageProps {
  proposal: PublicKey;
  author: PublicKey;
  postedAt: BN;
  replyTo?: PublicKey;  // ? makes `| undefined` union type redundant
  body: ChatMessageBody;
}
// export interface as in example 1, effectively a union of ChatMessageProps above, and ChatMessage below
export interface ChatMessage extends ChatMessageProps {}
export class ChatMessage extends BorshClass<ChatMessageProps> {
  // add properties and methods as usual, which will not to be passed in as args to the constructor
  accountType = GovernanceChatAccountType.ChatMessage;
}

TODO

• check if this is problematic with boolean arguments that are set to false when they are undefined i.e. this.isWritable = !!args.isWritable;
• double check nothing has been broken, maybe add some new tests

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[johnrees]

BorshClass is just a simple helper class that assigns the keys from the args object to properties on the instance

[johnrees]

this could probably be a decorator tbh but I didn't want to get too weird with it yet