cli: implement local download fallback

Implements an automatic local download fallback, similar to SSH
(cc @roblourens). If the initial download results in an error, either
in making the request or a 5xx, it'll try to fall back to making the
request locally and streaming it over the tunnel.

This abstracts the request client behing a "SimpleHttp" trait which
either uses to the native reqwest or uses the 'delegated' mode over the
socket.

cli/src/util/io.rs

@@ -2,7 +2,7 @@
  *  Copyright (c) Microsoft Corporation. All rights reserved.
  *  Licensed under the MIT License. See License.txt in the project root for license information.
  *--------------------------------------------------------------------------------------------*/
-use std::io;
+use std::{io, task::Poll};
 
 use tokio::io::{AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt};
 
@@ -57,3 +57,41 @@ where
 
 	Ok(bytes_so_far)
 }
+
+/// Helper used when converting Future interfaces to poll-based interfaces.
+/// Stores excess data that can be reused on future polls.
+#[derive(Default)]
+pub(crate) struct ReadBuffer(Option<(Vec<u8>, usize)>);
+
+impl ReadBuffer {
+	/// Removes any data stored in the read buffer
+	pub fn take_data(&mut self) -> Option<(Vec<u8>, usize)> {
+		self.0.take()
+	}
+
+	/// Writes as many bytes as possible to the readbuf, stashing any extra.
+	pub fn put_data(
+		&mut self,
+		target: &mut tokio::io::ReadBuf<'_>,
+		bytes: Vec<u8>,
+		start: usize,
+	) -> Poll<std::io::Result<()>> {
+		if bytes.is_empty() {
+			self.0 = None;
+			// should not return Ok(), since if nothing is written to the target
+			// it signals EOF. Instead wait for more data from the source.
+			return Poll::Pending;
+		}
+
+		if target.remaining() >= bytes.len() - start {
+			target.put_slice(&bytes[start..]);
+			self.0 = None;
+		} else {
+			let end = start + target.remaining();
+			target.put_slice(&bytes[start..end]);
+			self.0 = Some((bytes, end));
+		}
+
+		Poll::Ready(Ok(()))
+	}
+}