Force users to construct the Prisma Client themselves

[Weakky]

Description

It's currently not possible for a user to construct the Prisma Client and pass it to the nexus-plugin-prisma.

This feature will require a plugin configuration system first (#297).

[Weakky]

Given that we took the decision to always bundle the prisma deps, I think we can close this. It's already possible to construct it manually, but we won't force users to do so.