Support D1 Sessions API for global read replication

[juanferreras]

Cloudflare recently released global read replication for D1 (🎊 ) but it requires tagging all queries within a session to assure read-after-write consistency.

https://developers.cloudflare.com/d1/best-practices/read-replication/#replica-lag-and-consistency-model

I don't think a similar concept exists in Kysely, but we could potentially add a new session? argument to the constructor, and if used, .executeQuery would run against the const session = env.DB.withSession(...) instead of env.DB.

I think we might need to also expose some way of returning session.getBookmark(), and then users would need to:

1. Make sure to re-initialize on every request and not share a global singleton/etc.
2. Handle storing the resulting bookmark in an HTTP header and reading from it before initializing in their own code