fix(remote_storage): buffering, by using streams for upload and download (#5446)

There is double buffering in remote_storage and in pageserver for 8KiB
in using `tokio::io::copy` to read `BufReader<ReaderStream<_>>`.

Switches downloads and uploads to use `Stream<Item =
std::io::Result<Bytes>>`. Caller and only caller now handles setting up
buffering. For reading, `Stream<Item = ...>` is also a `AsyncBufRead`,
so when writing to a file, we now have `tokio::io::copy_buf` reading
full buffers and writing them to `tokio::io::BufWriter` which handles
the buffering before dispatching over to `tokio::fs::File`.

Additionally implements streaming uploads for azure. With azure
downloads are a bit nicer than before, but not much; instead of one huge
vec they just hold on to N allocations we got over the wire.

This PR will also make it trivial to switch reading and writing to
io-uring based methods.

Cc: #5563.

Cargo.lock

@@ -3864,6 +3864,7 @@ dependencies = [
  "bytes",
  "camino",
  "camino-tempfile",
+ "futures",
  "futures-util",
  "http-types",
  "hyper",
@@ -4291,6 +4292,7 @@ dependencies = [
  "tokio-io-timeout",
  "tokio-postgres",
  "tokio-stream",
+ "tokio-util",
  "toml_edit",
  "tracing",
  "url",
@@ -5220,6 +5222,7 @@ checksum = "5419f34732d9eb6ee4c3578b7989078579b7f039cbbb9ca2c4da015749371e15"
 dependencies = [
  "bytes",
  "futures-core",
+ "futures-io",
  "futures-sink",
  "pin-project-lite",
  "tokio",