feat(reader): add support for read replica

[johannsonic]

Hey!

We had a big problem in our production setup where all raw prisma-kysely queries hit our writer db, even for read-only operations. With this proposed solution, one can pass a configuration object to the extension and opt-in to use the replica instance for read queries.

I already used this implementation in our prod setup and our writer instance immediately cooled down :)

Johann

[tylerjbainbridge]

Would love to see this landed :)

[eoin-obrien]

Hi, and thanks for using prisma-extension-kysely!

The extension should work with read replicas as-is, but the order the extensions are applied in (and the way the read replicas are created) can result in the primary Kysely instance being shared across the .$primary().$kysely and .$replica().$kysely.

I've added some documentation on it to the readme if you'd like to check it out and see if it works properly when run like that.