New sessions driver config API

[florian-lefebvre]

Currently the sessions driver config is really tied to unstorage. The goal of this task is to introduce a new way to provide drivers, not unstorage specific:

-import { defineConfig } from 'astro/config'
+import { defineConfig, sessionDrivers } from 'astro/config'

export default defineConfig({
  session: {
-    driver: 'redis',
-    options: {
-      url: process.env.REDIS_URL
-    },
+    driver: sessionDrivers.redis({
+      url: process.env.REDIS_URL
+    }),
  }
})

This would remain backward compatible in v6 by simply deprecating options and driver as strings.