Plugins: Refactor call resource API handling (#67234)

Moving call resource API stream handling within plugin management as a utility/wrapper. 

Closes #66889

Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>

pkg/plugins/httpresponsesender/http_response_sender.go

@@ -0,0 +1,65 @@
+package httpresponsesender
+
+import (
+	"errors"
+	"fmt"
+	"net/http"
+	"net/textproto"
+
+	"github.com/grafana/grafana-plugin-sdk-go/backend"
+)
+
+// HTTPResponseSender implements backend.CallResourceResponseSender and
+// writes an HTTP response using an http.ResponseWriter given received
+// backend.CallResourceResponse(s).
+type HTTPResponseSender struct {
+	processedStreams int
+	w                http.ResponseWriter
+}
+
+// New creates a new HTTPResponseSender.
+func New(w http.ResponseWriter) *HTTPResponseSender {
+	if w == nil {
+		panic("response writer cannot be nil")
+	}
+
+	return &HTTPResponseSender{
+		w: w,
+	}
+}
+
+func (s *HTTPResponseSender) Send(resp *backend.CallResourceResponse) error {
+	if resp == nil {
+		return errors.New("resp cannot be nil")
+	}
+
+	// Expected that headers and status are only part of first stream
+	if s.processedStreams == 0 {
+		for k, values := range resp.Headers {
+			// Convert the keys to the canonical format of MIME headers.
+			// This ensures that we can safely add/overwrite headers
+			// even if the plugin returns them in non-canonical format
+			// and be sure they won't be present multiple times in the response.
+			k = textproto.CanonicalMIMEHeaderKey(k)
+
+			for _, v := range values {
+				s.w.Header().Add(k, v)
+			}
+		}
+
+		s.w.WriteHeader(resp.Status)
+	}
+
+	if _, err := s.w.Write(resp.Body); err != nil {
+		return fmt.Errorf("failed to write resource response: %v", err)
+	}
+
+	if flusher, ok := s.w.(http.Flusher); ok {
+		flusher.Flush()
+	}
+
+	s.processedStreams++
+	return nil
+}
+
+var _ backend.CallResourceResponseSender = &HTTPResponseSender{}