Backend plugins: Refactor to allow shared contract between core and external backend plugins (#25472)

Refactor to allow shared contract between core and external backend plugins 
allowing core backend data sources in Grafana to be implemented in same 
way as an external backend plugin.
Use v0.67.0 of sdk.
Add tests for verifying plugin is restarted when process is killed.
Enable strict linting for backendplugin packages

pkg/plugins/backendplugin/resource_response_stream.go

@@ -0,0 +1,57 @@
+package backendplugin
+
+import (
+	"context"
+	"errors"
+	"io"
+
+	"github.com/grafana/grafana-plugin-sdk-go/backend"
+)
+
+func newCallResourceResponseStream(ctx context.Context) *callResourceResponseStream {
+	return &callResourceResponseStream{
+		ctx:    ctx,
+		stream: make(chan *backend.CallResourceResponse),
+	}
+}
+
+type callResourceResponseStream struct {
+	ctx    context.Context
+	stream chan *backend.CallResourceResponse
+	closed bool
+}
+
+func (s *callResourceResponseStream) Send(res *backend.CallResourceResponse) error {
+	if s.closed {
+		return errors.New("Cannot send to a closed stream")
+	}
+
+	select {
+	case <-s.ctx.Done():
+		return errors.New("cancelled")
+	case s.stream <- res:
+		return nil
+	}
+}
+
+func (s *callResourceResponseStream) Recv() (*backend.CallResourceResponse, error) {
+	select {
+	case <-s.ctx.Done():
+		return nil, s.ctx.Err()
+	case res, ok := <-s.stream:
+		if !ok {
+			return nil, io.EOF
+		}
+		return res, nil
+	}
+}
+
+func (s *callResourceResponseStream) Close() error {
+	if s.closed {
+		return errors.New("Cannot close a closed stream")
+	}
+
+	close(s.stream)
+	s.closed = true
+	return nil
+}