To support AsyncRead-only requests, we need to return Option<u64> from SeekableStream::len()

[heaths]

If Storage wants to make it possible for customers to only upload a "stream" supporting futures::io::AsyncRead, we need to redesign SeekableStream::len() and is_empty() to return an Option<T>. This is because we cannot get the size using AsyncRead alone. We need AsyncSeek support and it has to be rewindable (which could fail).

One idea is to make the ReadStream in #4067 a builder like the FileStream I added in #4057 where with_buffer_size() can be called to specify a size, or - with an implementation specific to AsyncSeek being implemented.

One idea is something like,

pub struct ReadStream<R> {
    reader: R,
    len: Option<u64>,
    buffer_size: usize,
}

// We'd probably want an actual builder here so it's final, but just as an example...
impl<R: AsyncRead + Unpin + Send + Sync> ReadStream<R> {
    pub fn new(reader: R) -> Self {
        todo!()
    }
    pub fn with_buffer_size(self, buffer_size: usize) -> Self {
        todo!()
    }
    pub fn with_len(self, len: u64) -> Self {
        todo!()
    }
}

impl<R: AsyncRead + AsyncSeek + Unpin + Send + Sync> ReadStream<R> {
    pub async fn determine_size(self) -> Result<Self> {
        todo!()
    }
}