DataSources: Add datasource fetching + querying interface (#80749)

* first pass

* separate oss + enterprise

* tidy things up

* add ctx

* fix tests

* use standalone svcs

* mv plugin context provide

* fix wire

* fix import

pkg/registry/apis/datasource/plugincontext.go

@@ -0,0 +1,11 @@
+package datasource
+
+import (
+	"context"
+
+	"github.com/grafana/grafana-plugin-sdk-go/backend"
+)
+
+type PluginContextProvider interface {
+	PluginContextForDataSource(ctx context.Context, pluginID, name string) (backend.PluginContext, error)
+}