[Auth] Implement POST handlers for login, signup, etc. on dbAuth

[dac09]

The dbAuth middleware will need to "handoff" requests to the DbAuthHandler whenever it intercepts a request for login, signup, etc.

The dbAuthHandler has to have the capability of parsing a Fetch API request, and responding so that the middleware can then use the output.

In essence the highlighted section of the middleware:

export const createDbAuthMiddleware = (/*...*/) => {
  return async (req: MiddlewareRequest) => {
    const res = MiddlewareResponse.next()

    if (req.method === 'POST') {
  // 👇 Middleware handoff to dbAuthHandler 👇
      const output = await dbAuthHandler(req)