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/ifaces.go

@@ -3,6 +3,7 @@ package plugins
 import (
 	"context"
 	"io/fs"
+	"time"
 
 	"github.com/grafana/grafana-plugin-sdk-go/backend"
 
@@ -30,6 +31,16 @@ type PluginSource interface {
 	DefaultSignature(ctx context.Context) (Signature, bool)
 }
 
+type FileStore interface {
+	// File retrieves a plugin file.
+	File(ctx context.Context, pluginID, filename string) (*File, error)
+}
+
+type File struct {
+	Content []byte
+	ModTime time.Time
+}
+
 type CompatOpts struct {
 	GrafanaVersion string
 	OS             string