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/any_nil.go

@@ -0,0 +1,69 @@
+package jsoniter
+
+type nilAny struct {
+	baseAny
+}
+
+func (any *nilAny) LastError() error {
+	return nil
+}
+
+func (any *nilAny) ValueType() ValueType {
+	return NilValue
+}
+
+func (any *nilAny) MustBeValid() Any {
+	return any
+}
+
+func (any *nilAny) ToBool() bool {
+	return false
+}
+
+func (any *nilAny) ToInt() int {
+	return 0
+}
+
+func (any *nilAny) ToInt32() int32 {
+	return 0
+}
+
+func (any *nilAny) ToInt64() int64 {
+	return 0
+}
+
+func (any *nilAny) ToUint() uint {
+	return 0
+}
+
+func (any *nilAny) ToUint32() uint32 {
+	return 0
+}
+
+func (any *nilAny) ToUint64() uint64 {
+	return 0
+}
+
+func (any *nilAny) ToFloat32() float32 {
+	return 0
+}
+
+func (any *nilAny) ToFloat64() float64 {
+	return 0
+}
+
+func (any *nilAny) ToString() string {
+	return ""
+}
+
+func (any *nilAny) WriteTo(stream *Stream) {
+	stream.WriteNil()
+}
+
+func (any *nilAny) Parse() *Iterator {
+	return nil
+}
+
+func (any *nilAny) GetInterface() interface{} {
+	return nil
+}