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/pool.go

@@ -0,0 +1,42 @@
+package jsoniter
+
+import (
+	"io"
+)
+
+// IteratorPool a thread safe pool of iterators with same configuration
+type IteratorPool interface {
+	BorrowIterator(data []byte) *Iterator
+	ReturnIterator(iter *Iterator)
+}
+
+// StreamPool a thread safe pool of streams with same configuration
+type StreamPool interface {
+	BorrowStream(writer io.Writer) *Stream
+	ReturnStream(stream *Stream)
+}
+
+func (cfg *frozenConfig) BorrowStream(writer io.Writer) *Stream {
+	stream := cfg.streamPool.Get().(*Stream)
+	stream.Reset(writer)
+	return stream
+}
+
+func (cfg *frozenConfig) ReturnStream(stream *Stream) {
+	stream.out = nil
+	stream.Error = nil
+	stream.Attachment = nil
+	cfg.streamPool.Put(stream)
+}
+
+func (cfg *frozenConfig) BorrowIterator(data []byte) *Iterator {
+	iter := cfg.iteratorPool.Get().(*Iterator)
+	iter.ResetBytes(data)
+	return iter
+}
+
+func (cfg *frozenConfig) ReturnIterator(iter *Iterator) {
+	iter.Error = nil
+	iter.Attachment = nil
+	cfg.iteratorPool.Put(iter)
+}