Expose ndc_sdk (#554)

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### What

Allows us to talk about the right underlying models when importing
`ndc-postgres` as a library.

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obvious)? -->

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### How

Export `ndc_sdk as sdk` in `lib.rs` for `ndc-postgres` connector.

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steps)? -->

crates/connectors/ndc-postgres/src/lib.rs

@@ -9,3 +9,6 @@ pub mod mutation;
 pub mod query;
 pub mod schema;
 pub mod state;
+
+// we expose the sdk used for this connector so that we are able to use it in multitenant
+pub use ndc_sdk as sdk;