WIP: Spawn backend plugins v2 (#19835)

* WIP: Spawn backend plugins v2

* Add test for plugin version

* Fix support for SDK plugins

Co-authored-by: Kyle Brandt <kyle@kbrandt.com>
Co-authored-by: Marcus Olsson <olsson.e.marcus@gmail.com>
Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

* WIP: Draft PR for fork of V2 sdk / bi-directional support (#19890)

* temporary use export-datasource-plugin branch of grafana-plugin-sdk

* fix failing test

* remove debug (spew) lines

* misc cleanup

* add expressions feature toggle

* use latest grafana-plugin-sdk-go

vendor/github.com/mattetti/filebuffer/README.md

@@ -0,0 +1,10 @@
+# filebuffer
+
+filebuffer is a package implementing a few file-like interfaces such as `io.Reader`, `io.ReaderAt`, `io.Seeker` and more.
+The implementation is backed by a byte buffer and the main purpose is to have in-memory alternative to using an io.File.
+
+More information can be found on the [GoDoc page](https://godoc.org/github.com/mattetti/filebuffer).
+
+[![GoDoc](http://godoc.org/github.com/mattetti/filebuffer?status.svg)](http://godoc.org/github.com/mattetti/filebuffer)
+
+[![Build Status](https://travis-ci.org/mattetti/filebuffer.png)](https://travis-ci.org/mattetti/filebuffer)