fix: express session type compatibility

[ojeytonwilliams]

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

This PR was motivated by the current need to assert that the Store types are any due to the incompatibility of @fastify/session's types with express-session's. e.g.

server.register(fastifySession, {
  store: new RedisStore({
    client: redisClient,
  }) as any,

I tried to keep the changes to a minimum, but it was necessary to alter the callbacks types and to change precisely what was in the various Session types.

The callback needed updating because express-session's callbacks all define their errors as any and because we can't guarantee that a session is passed to the callback.

The Session types needed updating because the stores are only compatible with express-session's SessionData rather than the full set of properties that a fastify session object has.

I wasn't able to run the benchmarks locally, this PR only modifies types, so I hope that's okay.

[mcollina]

lgtm