feat: add async mutation support

[PaulBrachmann]

This PR enables support for RelayMutations to be async (meaning they can return a Promise). Currently, such mutations lead to null returns.

E.g., mutations such as these resolve properly with the proposed changes:

  @RelayMutation(() => DeleteUserPayload)
  async deleteUser(@InputArg(() => DeleteUserInput) input: DeleteUserInput) {
    await this.usersService.remove(input.id.toString());
    return new DeleteUserPayload(input.id);
  }

[leesiongchan]

Kinda bummer without async support! This could be super useful!

[rogerballard]

Hey @PaulBrachmann thanks for your submission. I completely agree that async functionality would be really useful here.

I will look at getting this PR merged once I have fixed the development workflow.