sessionConfig.valid: make optional (#18748)

`koa-session` does not actually require this validator and will provide a default behavior if its not supplied.

see: https://github.com/koajs/session/blob/cb6d2c8448f21f6cf42efdb031798c9608f05602/lib/context.js#L152-L167

types/koa-session/index.d.ts

@@ -59,7 +59,7 @@ declare namespace session {
         /**
          * Hook: valid session value before use it
          */
-        valid(...rest: any[]): void;
+        valid?(...rest: any[]): void;
 
         /**
          * Hook: before save session

types/koa-session/koa-session-tests.ts

@@ -1,5 +1,5 @@
 import * as Koa from 'koa';
-import session = require('koa-session');
+import * as session from 'koa-session';
 
 const app = new Koa();