Add tryGetSession that doesn't create a new entry in a database to SessionStorage interface

[tats-u]

getSession always runs createSession when readData cannot find a corresponding entry.
Another method that does not run createSession in such a case is required when you develop a member-only application and expose its login page to the public.
If createSession is called too many times (e.g. by DoS attack), the performance of a database can be degraded, especially when you use only RDBMS and do not combine it with KVS NoSQL (e.g. Redis/Valkey).

  getSession: (
    cookieHeader?: string | null,
    options?: ParseOptions
  ) => Promise<Session<Data, FlashData>>;

vs

  tryGetSession: (
    cookieHeader?: string | null,
    options?: ParseOptions
  ) => Promise<Session<Data, FlashData> | null>;

As you might notice, it returns just null without creating new one when no entry.

This will not be a breaking change unless you use third party session libraries. However, an author of a third party session library have to modify their library to implement this method. Unfortunately JavaScript/TypeScript cannot provide default method implementations in an interface unlike Java or C#.