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
00940fcdb5
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

libs/proxy/postgres-protocol2/src/lib.rs

@@ -0,0 +1,78 @@
+//! Low level Postgres protocol APIs.
+//!
+//! This crate implements the low level components of Postgres's communication
+//! protocol, including message and value serialization and deserialization.
+//! It is designed to be used as a building block by higher level APIs such as
+//! `rust-postgres`, and should not typically be used directly.
+//!
+//! # Note
+//!
+//! This library assumes that the `client_encoding` backend parameter has been
+//! set to `UTF8`. It will most likely not behave properly if that is not the case.
+#![doc(html_root_url = "https://docs.rs/postgres-protocol/0.6")]
+#![warn(missing_docs, rust_2018_idioms, clippy::all)]
+
+use byteorder::{BigEndian, ByteOrder};
+use bytes::{BufMut, BytesMut};
+use std::io;
+
+pub mod authentication;
+pub mod escape;
+pub mod message;
+pub mod password;
+pub mod types;
+
+/// A Postgres OID.
+pub type Oid = u32;
+
+/// A Postgres Log Sequence Number (LSN).
+pub type Lsn = u64;
+
+/// An enum indicating if a value is `NULL` or not.
+pub enum IsNull {
+    /// The value is `NULL`.
+    Yes,
+    /// The value is not `NULL`.
+    No,
+}
+
+fn write_nullable<F, E>(serializer: F, buf: &mut BytesMut) -> Result<(), E>
+where
+    F: FnOnce(&mut BytesMut) -> Result<IsNull, E>,
+    E: From<io::Error>,
+{
+    let base = buf.len();
+    buf.put_i32(0);
+    let size = match serializer(buf)? {
+        IsNull::No => i32::from_usize(buf.len() - base - 4)?,
+        IsNull::Yes => -1,
+    };
+    BigEndian::write_i32(&mut buf[base..], size);
+
+    Ok(())
+}
+
+trait FromUsize: Sized {
+    fn from_usize(x: usize) -> Result<Self, io::Error>;
+}
+
+macro_rules! from_usize {
+    ($t:ty) => {
+        impl FromUsize for $t {
+            #[inline]
+            fn from_usize(x: usize) -> io::Result<$t> {
+                if x > <$t>::MAX as usize {
+                    Err(io::Error::new(
+                        io::ErrorKind::InvalidInput,
+                        "value too large to transmit",
+                    ))
+                } else {
+                    Ok(x as $t)
+                }
+            }
+        }
+    };
+}
+
+from_usize!(i16);
+from_usize!(i32);