proxy(tokio-postgres): refactor typeinfo query to occur earlier (#11993)

## Problem

For #11992 I realised we need to get the type info before executing the
query. This is important to know how to decode rows with custom types,
eg the following query:

```sql
CREATE TYPE foo AS ENUM ('foo','bar','baz');
SELECT ARRAY['foo'::foo, 'bar'::foo, 'baz'::foo] AS data;
```

Getting that to work was harder that it seems. The original
tokio-postgres setup has a split between `Client` and `Connection`,
where messages are passed between. Because multiple clients were
supported, each client message included a dedicated response channel.
Each request would be terminated by the `ReadyForQuery` message.

The flow I opted to use for parsing types early would not trigger a
`ReadyForQuery`. The flow is as follows:

```
PARSE ""    // parse the user provided query
DESCRIBE "" // describe the query, returning param/result type oids
FLUSH       // force postgres to flush the responses early

// wait for descriptions

  // check if we know the types, if we don't then
  // setup the typeinfo query and execute it against each OID:

  PARSE typeinfo    // prepare our typeinfo query
  DESCRIBE typeinfo
  FLUSH // force postgres to flush the responses early

  // wait for typeinfo statement

    // for each OID we don't know:
    BIND typeinfo
    EXECUTE
    FLUSH

    // wait for type info, might reveal more OIDs to inspect

  // close the typeinfo query, we cache the OID->type map and this is kinder to pgbouncer.
  CLOSE typeinfo 

// finally once we know all the OIDs:
BIND ""   // bind the user provided query - already parsed - to the user provided params
EXECUTE   // run the user provided query
SYNC      // commit the transaction
```

## Summary of changes

Please review commit by commit. The main challenge was allowing one
query to issue multiple sub-queries. To do this I first made sure that
the client could fully own the connection, which required removing any
shared client state. I then had to replace the way responses are sent to
the client, by using only a single permanent channel. This required some
additional effort to track which query is being processed. Lastly I had
to modify the query/typeinfo functions to not issue `sync` commands, so
it would fit into the desired flow above.

To note: the flow above does force an extra roundtrip into each query. I
don't know yet if this has a measurable latency overhead.

libs/proxy/postgres-protocol2/src/message/frontend.rs

@@ -25,6 +25,7 @@ where
     Ok(())
 }
 
+#[derive(Debug)]
 pub enum BindError {
     Conversion(Box<dyn Error + marker::Sync + Send>),
     Serialization(io::Error),
@@ -288,6 +289,12 @@ pub fn sync(buf: &mut BytesMut) {
     write_body(buf, |_| Ok::<(), io::Error>(())).unwrap();
 }
 
+#[inline]
+pub fn flush(buf: &mut BytesMut) {
+    buf.put_u8(b'H');
+    write_body(buf, |_| Ok::<(), io::Error>(())).unwrap();
+}
+
 #[inline]
 pub fn terminate(buf: &mut BytesMut) {
     buf.put_u8(b'X');