Blob download() result type should return stream

[jeremymeng]

the download() operation result type has

  ): (BodyParameter & {
    /** The media type of the body of the response. */
    @header("Content-Type")
    contentType: "application/octet-stream";

Our generated code today reutrns Promise<Uint8Array>, we should generate code that returns .asNodeStream() or asBrowserStream()

[timovv]

A couple of initial notes:

• This change: Fix binary response handling when response body is not valid UTF-8 #3572 is somewhat related; in that we changed binary response handling to use asNodeStream/asBrowserStream in the codegen. You could imagine just having the generated code return the StreamableMethod instead of converting it into a Uint8Array.
• There might be some breaking change concern here for other generated SDKs which are relying on these operations returnning Uint8Array so I will investigate the potential impact here.

[timovv]

Talked with @joheredi about this a little and we think we can mitigate the breaking change risk using @@clientOption to let people opt in to the streaming behavior.

[jeremymeng]

currently working around it by calling _downloadSend function directly