Plugins: Add inititialization stage to plugin loader pipeline (#72667)

* first pass

* migrate tests

* simplify

* fix comments

* fix linter

* nil checks

* remove comment

pkg/plugins/manager/pipeline/initialization/initialization.go

@@ -0,0 +1,67 @@
+package initialization
+
+import (
+	"context"
+
+	"github.com/grafana/grafana/pkg/plugins"
+	"github.com/grafana/grafana/pkg/plugins/config"
+	"github.com/grafana/grafana/pkg/plugins/log"
+)
+
+// Initializer is responsible for the Initialization stage of the plugin loader pipeline.
+type Initializer interface {
+	Initialize(ctx context.Context, ps []*plugins.Plugin) ([]*plugins.Plugin, error)
+}
+
+// InitializeFunc is the function used for the Initialize step of the Initialization stage.
+type InitializeFunc func(ctx context.Context, p *plugins.Plugin) (*plugins.Plugin, error)
+
+type Initialize struct {
+	cfg             *config.Cfg
+	initializeSteps []InitializeFunc
+	log             log.Logger
+}
+
+type Opts struct {
+	InitializeFuncs []InitializeFunc
+}
+
+// New returns a new Initialization stage.
+func New(cfg *config.Cfg, opts Opts) *Initialize {
+	if opts.InitializeFuncs == nil {
+		opts.InitializeFuncs = []InitializeFunc{}
+	}
+
+	return &Initialize{
+		cfg:             cfg,
+		initializeSteps: opts.InitializeFuncs,
+		log:             log.New("plugins.initialization"),
+	}
+}
+
+// Initialize will execute the Initialize steps of the Initialization stage.
+func (i *Initialize) Initialize(ctx context.Context, ps []*plugins.Plugin) ([]*plugins.Plugin, error) {
+	if len(i.initializeSteps) == 0 {
+		return ps, nil
+	}
+
+	var err error
+	initializedPlugins := make([]*plugins.Plugin, 0, len(ps))
+	for _, p := range ps {
+		var ip *plugins.Plugin
+		stepFailed := false
+		for _, init := range i.initializeSteps {
+			ip, err = init(ctx, p)
+			if err != nil {
+				stepFailed = true
+				i.log.Error("Could not initialize plugin", "pluginId", p.ID, "err", err)
+				break
+			}
+		}
+		if !stepFailed {
+			initializedPlugins = append(initializedPlugins, ip)
+		}
+	}
+
+	return initializedPlugins, nil
+}