[TS] Allow to specify success/error ApiResponse

[Andreyco]

Currenty, ApiResponse type is declared as:

type ApiResponse<T> = ApiErrorResponse<T> | ApiOkResponse<T>;

This means, both types for success and error is T

Allow end user to specify success/error types in following way:

// APISAUCE types
type ApiErrorResponse<T> = {
  ok: false,
  data: T
}

type ApiOkResponse < T > = {
  ok: true,
  data: T
}

type ApiResponse<O, E = O> = ApiErrorResponse<E> | ApiOkResponse<O>;

// Userland types
type Ok = {
  firstName: string
}

type Err = {
  message: string
}

// Example of success/error responses
// Success response
const responseOk: ApiResponse<Ok, Err> = {
  ok: true,
  data: {
    firstName: 'Andrej'
  }
} 

// Error response
const responseErr: ApiResponse<Ok, Err> = {
  ok: false,
  data: {
    message: 'xx'
  }
}

// Example of `get` request
const get = () => {
  return {} as ApiResponse<Ok, Err>
} 

const response = get()

if (response.ok === true) {
  console.log(response.data.firstName)
} else {
  console.log(response.data.message)
}

[ryanlntn]

Addressed with #219