JavaScript: Cognito - Add type annotation on authService.ts

[NathanCYee]

When running npm run build on the project, the build fails due to the following error:

src/authService.ts:26:45 - error TS2769: No overload matches this call.
  Overload 1 of 2, '(input: InitiateAuthCommandInput): InitiateAuthCommand', gave the following error.
    Argument of type '{ AuthFlow: string; ClientId: string; AuthParameters: { USERNAME: string; PASSWORD: string; }; }' is not assignable to parameter of type 'InitiateAuthCommandInput'.
      Types of property 'AuthFlow' are incompatible.
        Type 'string' is not assignable to type 'AuthFlowType'.
  Overload 2 of 2, '(__0_0: InitiateAuthCommandInput): InitiateAuthCommand', gave the following error.
    Argument of type '{ AuthFlow: string; ClientId: string; AuthParameters: { USERNAME: string; PASSWORD: string; }; }' is not assignable to parameter of type 'InitiateAuthCommandInput'.
      Types of property 'AuthFlow' are incompatible.
        Type 'string' is not assignable to type 'AuthFlowType'.

26     const command = new InitiateAuthCommand(params);

By adding type annotations, TypeScript does not complain on the line. Added annotations on every other API call as well for consistency.