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/build.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+set -e
+set -x
+
+if [ ! -d /tmp/build-golang/src/github.com/json-iterator ]; then
+    mkdir -p /tmp/build-golang/src/github.com/json-iterator
+    ln -s $PWD /tmp/build-golang/src/github.com/json-iterator/go
+fi
+export GOPATH=/tmp/build-golang
+go get -u github.com/golang/dep/cmd/dep
+cd /tmp/build-golang/src/github.com/json-iterator/go
+exec $GOPATH/bin/dep ensure -update