add a Rust client for Pageserver management API (#6127)

Part of getpage@lsn benchmark epic:
https://github.com/neondatabase/neon/issues/5771

This PR moves the control plane's spread-all-over-the-place client for
the pageserver management API into a separate module within the
pageserver crate.

I need that client to be async in my benchmarking work, so, this PR
switches to the async version of `reqwest`.
That is also the right direction generally IMO.

The switch to async in turn mandated converting most of the
`control_plane/` code to async.

Note that some of the client methods should be taking `TenantShardId`
instead of `TenantId`, but, none of the callers seem to be
sharding-aware.
Leaving that for another time:
https://github.com/neondatabase/neon/issues/6154

libs/postgres_connection/src/lib.rs

@@ -163,8 +163,18 @@ impl PgConnectionConfig {
     }
 
     /// Connect using postgres protocol with TLS disabled.
-    pub fn connect_no_tls(&self) -> Result<postgres::Client, postgres::Error> {
-        postgres::Config::from(self.to_tokio_postgres_config()).connect(postgres::NoTls)
+    pub async fn connect_no_tls(
+        &self,
+    ) -> Result<
+        (
+            tokio_postgres::Client,
+            tokio_postgres::Connection<tokio_postgres::Socket, tokio_postgres::tls::NoTlsStream>,
+        ),
+        postgres::Error,
+    > {
+        self.to_tokio_postgres_config()
+            .connect(postgres::NoTls)
+            .await
     }
 }