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/Gopkg.lock

@@ -0,0 +1,21 @@
+# This file is autogenerated, do not edit; changes may be undone by the next 'dep ensure'.
+
+
+[[projects]]
+  name = "github.com/modern-go/concurrent"
+  packages = ["."]
+  revision = "e0a39a4cb4216ea8db28e22a69f4ec25610d513a"
+  version = "1.0.0"
+
+[[projects]]
+  name = "github.com/modern-go/reflect2"
+  packages = ["."]
+  revision = "4b7aa43c6742a2c18fdef89dd197aaae7dac7ccd"
+  version = "1.0.1"
+
+[solve-meta]
+  analyzer-name = "dep"
+  analyzer-version = 1
+  inputs-digest = "ea54a775e5a354cb015502d2e7aa4b74230fc77e894f34a838b268c25ec8eeb8"
+  solver-name = "gps-cdcl"
+  solver-version = 1