Plugins: Use grafana-plugin-sdk-go v0.5.0 (#21116)

* fix dispense

* plugin loading refactor

Moves common functionality of registering, starting,
stopping and restarting backend plugins to
backendplugin package.

* simplify plugin descriptors

* target sdk v0.5.0

Co-authored-by: Kyle Brandt <kyle@kbrandt.com>

vendor/github.com/json-iterator/go/go.mod

@@ -0,0 +1,11 @@
+module github.com/json-iterator/go
+
+go 1.12
+
+require (
+	github.com/davecgh/go-spew v1.1.1
+	github.com/google/gofuzz v1.0.0
+	github.com/modern-go/concurrent v0.0.0-20180228061459-e0a39a4cb421
+	github.com/modern-go/reflect2 v0.0.0-20180701023420-4b7aa43c6742
+	github.com/stretchr/testify v1.3.0
+)