Improve `WorkerThreadManager` type inference

[zone117x]

The WorkerThreadManager implementation does not understand conditional return types for the .exec(...) function. Right now the type inference code extracts the args and response type separately, e.g.:

type WorkerPoolModuleInterface<TArgs extends unknown[], TResp> = {
  workerModule: NodeJS.Module;
  processTask: (...args: TArgs) => Promise<TResp> | TResp; // <-- any type interface involving both TArgs and TResp are broken here
}

declare class WorkerThreadManager<TArgs extends unknown[], TResp> {
  static init<TArgs extends unknown[], TResp>(workerModule: WorkerPoolModuleInterface<TArgs, TResp>, opts?: {
    workerCount?: number;
  }): Promise<WorkerThreadManager<TArgs, TResp>>;
  constructor(workerModule: WorkerPoolModuleInterface<TArgs, TResp>, opts?: {
    workerCount?: number;
  });
  exec(...args: TArgs): Promise<TResp>;
}

If the worker module defines conditional return types like the following, then the .exec(...) function incorrectly only allows the first possible input and output types.

type TaskArg =
  | { kind: 'block'; msg: CoreNodeNakamotoBlockMessage }
  | { kind: 'chunk'; msg: StackerDbChunk };

type TaskResult<T extends TaskArg> = T['kind'] extends 'block'
  ? { kind: 'block'; result: ReturnType<typeof parseNakamotoBlockMsg> }
  : { kind: 'chunk'; result: ReturnType<typeof parseStackerDbChunk> };

export function processTask<T extends TaskArg>(args: T): TaskResult<T> {
  if (args.kind === 'block') {
    return {
      kind: 'block',
      result: parseNakamotoBlockMsg(args.msg),
    } as TaskResult<T>;
  } else {
    return {
      kind: 'chunk',
      result: parseStackerDbChunk(args.msg),
    } as TaskResult<T>;
  }
}

Originally posted by @zone117x in hirosystems/signer-metrics-api#85 (comment)