koa [feat] Support Web (WHATWG) stream, Blob and Response

Describe the feature

Currently response.body= or ctx.body= support buffers, strings and Nodejs streams. I suggest we also add support to Web stream, Blob and Response (currently just fallback to json which break users expectation), maybe also all async iterables, Nodejs already have those APIs in global.

This is my workaround, it works but it's better to support them in the core.

app.use(async (ctx, next) => {
    await next();
    // Note: need to clear type before set body, or the type might always be json
    // See Koa impl: https://github.com/koajs/koa/blob/dbf4b8f41286befd53dfd802740f2021441435bf/lib/response.js#L134-L190
    if (ctx.body instanceof ReadableStream) {
        ctx.type = "";
        ctx.body = Readable.fromWeb(ctx.body);
    } else if (ctx.body instanceof Blob) {
        const blob = ctx.body;
        ctx.type = blob.type;
        ctx.length = blob.size;
        ctx.body = Readable.fromWeb(blob.stream());
    } else if (ctx.body instanceof Response) {
        const response = ctx.body;
        ctx.status = response.status;
        ctx.type = "";
        // Note: don't clear all headers so the headers added by middlewares still effective
        for (const [name, value] of response.headers) ctx.set(name, value);
        if (response.redirected) ctx.redirect(response.url);
        else ctx.body = Readable.fromWeb(response.body);
    }
    // maybe we could also support all async iterables
    // else if (ctx.body[Symbol.asyncIterator]) {
    //   ctx.type = ""; ctx.body = Readable.from(ctx.body);
    // }
});