[proxy] Forward compute connection params to client

This fixes all kinds of problems related to missing params,
like broken timestamps (due to `integer_datetimes`).

This solution is not ideal, but it will help. Meanwhile,
I'm going to dedicate some time to improving connection machinery.

Note that this **does not** fix problems with passing certain parameters
in a reverse direction, i.e. **from client to compute**. This is a
separate matter and will be dealt with in an upcoming PR.

proxy/src/proxy/tests.rs

@@ -139,7 +139,7 @@ async fn dummy_proxy(
 
     stream
         .write_message_noflush(&Be::AuthenticationOk)?
-        .write_message_noflush(&BeParameterStatusMessage::encoding())?
+        .write_message_noflush(&Be::CLIENT_ENCODING)?
         .write_message(&BeMessage::ReadyForQuery)
         .await?;