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/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "postgres-protocol2"
+version = "0.1.0"
+edition = "2018"
+license = "MIT/Apache-2.0"
+
+[dependencies]
+base64 = "0.20"
+byteorder.workspace = true
+bytes.workspace = true
+fallible-iterator.workspace = true
+hmac.workspace = true
+md-5 = "0.10"
+memchr = "2.0"
+rand.workspace = true
+sha2.workspace = true
+stringprep = "0.1"
+tokio = { workspace = true, features = ["rt"] }
+
+[dev-dependencies]
+tokio = { workspace = true, features = ["full"] }