Plugins: Add file store abstraction for handling plugin files (#65432)

* add file store

* fix markdown fetch bug

* add markdown tests

* fix var name

pkg/plugins/manager/filestore/fs.go

@@ -0,0 +1,54 @@
+package filestore
+
+import (
+	"context"
+	"io"
+
+	"github.com/grafana/grafana/pkg/plugins"
+	"github.com/grafana/grafana/pkg/plugins/log"
+	"github.com/grafana/grafana/pkg/plugins/manager/registry"
+)
+
+type Service struct {
+	pluginRegistry registry.Service
+	log            log.Logger
+}
+
+func ProvideService(pluginRegistry registry.Service) *Service {
+	return &Service{
+		pluginRegistry: pluginRegistry,
+		log:            log.New("plugin.fs"),
+	}
+}
+
+func (s *Service) File(ctx context.Context, pluginID, filename string) (*plugins.File, error) {
+	if p, exists := s.pluginRegistry.Plugin(ctx, pluginID); exists {
+		f, err := p.File(filename)
+		if err != nil {
+			return nil, err
+		}
+		defer func() {
+			err = f.Close()
+			if err != nil {
+				s.log.Error("Could not close plugin file", "pluginID", p.ID, "file", filename)
+			}
+		}()
+
+		b, err := io.ReadAll(f)
+		if err != nil {
+			return nil, err
+		}
+
+		fi, err := f.Stat()
+		if err != nil {
+			return nil, err
+		}
+
+		return &plugins.File{
+			Content: b,
+			ModTime: fi.ModTime(),
+		}, nil
+	} else {
+		return nil, plugins.ErrPluginNotInstalled
+	}
+}