Add helper download.InMemory for downloading files without writing to…

… disk

Currently we have separate code that downloads the release-info.json file
and separate code that downloads the signed bundle hashes file, both  the
files are downloaded in memory this helper will unify that code and we'll
use this helper to download both of these files

It sets the http User-Agent to `crc/<version>` which is currently used for
downloading the file release-info.json

pkg/download/download.go

@@ -3,6 +3,8 @@ package download
 import (
 	"crypto/sha256"
 	"encoding/hex"
+	"fmt"
+	"io"
 	"net/http"
 	"net/url"
 	"os"
@@ -84,6 +86,30 @@ func Download(uri, destination string, mode os.FileMode, sha256sum []byte) (stri
 	return filename, nil
 }
 
+// Download takes an URL and the User-Agent string to use in the http request
+// it takes  http.RoundTripper as the third argument to be used by the client
+func InMemory(url, userAgent string, transport http.RoundTripper) (io.ReadCloser, error) {
+	client := grab.NewClient()
+	client.HTTPClient = &http.Client{Transport: transport}
+
+	httpReq, err := http.NewRequest(http.MethodGet, url, nil)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create http request: %w", err)
+	}
+	httpReq.Header.Set("User-Agent", userAgent)
+
+	grabReq, err := grab.NewRequest("", "")
+	if err != nil {
+		return nil, fmt.Errorf("failed to create grab request: %w", err)
+	}
+	grabReq.HTTPRequest = httpReq
+	// do not write the downloaded file to disk
+	grabReq.NoStore = true
+
+	rsp := client.Do(grabReq)
+	return rsp.Open()
+}
+
 type RemoteFile struct {
 	URI       string
 	sha256sum string