Add support for typed session data

[moeriki]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

I'd like to type the data I put in and get out of the session.

Motivation

I don't like type-casting any on every session.get().

Example

// fastify-secure-session/index.d.ts
export interface Session {
  // …
  get<Key extends keyof SessionData>(key: Key): SessionData[Key] | undefined;
  set<Key extends keyof SessionData>(key: Key, value: SessionData[Key] | undefined): void;
}

export interface SessionData {
  [key: string]: any;
}

# user code
declare module 'fastify-secure-session' {
  interface SessionData {
    codeVerifier: string;
    tokenSet: TokenSet;
  }
}

[climba03003]

PR is welcome. Remember to add an unit test.