chore(proxy): vendor a subset of rust-postgres (#9930)

Our rust-postgres fork is getting messy. Mostly because proxy wants more
control over the raw protocol than tokio-postgres provides. As such,
it's diverging more and more. Storage and compute also make use of
rust-postgres, but in more normal usage, thus they don't need our crazy
changes.

Idea: 
* proxy maintains their subset
* other teams use a minimal patch set against upstream rust-postgres

Reviewing this code will be difficult. To implement it, I
1. Copied tokio-postgres, postgres-protocol and postgres-types from
https://github.com/neondatabase/rust-postgres/tree/00940fcdb57a8e99e805297b75839e7c4c7b1796
2. Updated their package names with the `2` suffix to make them compile
in the workspace.
3. Updated proxy to use those packages
4. Copied in the code from tokio-postgres-rustls 0.13 (with some patches
applied https://github.com/jbg/tokio-postgres-rustls/pull/32
https://github.com/jbg/tokio-postgres-rustls/pull/33)
5. Removed as much dead code as I could find in the vendored libraries
6. Updated the tokio-postgres-rustls code to use our existing channel
binding implementation

proxy/src/context/mod.rs

@@ -414,6 +414,7 @@ impl RequestContextInner {
                     outcome,
                 });
         }
+
         if let Some(tx) = self.sender.take() {
             // If type changes, this error handling needs to be updated.
             let tx: mpsc::UnboundedSender<RequestData> = tx;