Add legacy encryption driver for backward compatibility

Adds a new legacy encryption driver that maintains compatibility with the AdonisJS v6 encryption format.

Usage:

import { defineConfig, drivers } from '@adonisjs/core/encryption'

export default defineConfig({
  default: 'legacy',
  list: {
    legacy: drivers.legacy({
      keys: [env.get('APP_KEY')],
    }),
  },
})

Migration path:

Use this driver to decrypt existing encrypted values from AdonisJS v6, then re-encrypt them with a modern driver (aes256gcm recommended) for improved security.