Pull out a configuration library.

[SamirTalwar]

What

We require a configuration library so that we can re-use configuration types and functions in both a CLI plugin and a server-side connector build service.

This has been done with minimal effort.

Further work is required to minimize its surface area. Specifically, I believe we should:

1. reduce the number of crates depending on this one,
2. remove the dependency from "ndc-postgres-configuration" on "query-engine-sql" (it's only used to pull in IsolationLevel), and
3. refactor to reduce the number of exposed types and functions as much as possible.

I have not done any of these things in this changeset to make it easier to review, and because I didn't want reviews of this extraction to get bogged down in whether we should do the above or not.

How

I moved ndc-postgres/src/configuration.rs and everything in ndc-postgres/src/configuration to their own crate, imaginatively named "ndc-postgres-configuration".

I then re-exported what was necessary from lib.rs, and changed imports in several places.

[plcplc]

LGTM