[Auth] Implement dbAuth middleware

[dac09]

Implement dbAuth middleware in a new package:

import { createDbAuthMiddleware } from '@redwoodjs/auth-dbauth-middleware'

so we can use in entry.server like so:

export const registerMiddleware = () => {
  const dbAuthMiddleware = createDbAuthMiddleware({
    cookieName,
    dbAuthHandler,
    getCurrentUser,
  })
  return [dbAuthMiddleware]
}