Add inmemory.Download for downloading files without writing to disk

this helper is put in a separate package pkg/download/inmemory to avoid
import cycles which occurs due to the network package being imported in
the pkg/download package, the cycle is network->ssh->constants->version
as a result we'll not be able to import pkg/download in pkg/crc/version

currently the helper is used for downloading release-info.json file and
the signed bundle hashes file sha256sum.txt.sig

pkg/download/inmemory/inmemory.go

@@ -0,0 +1,33 @@
+package inmemory
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+
+	"github.com/cavaliergopher/grab/v3"
+)
+
+// 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 Download(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()
+}