Implement registries for Kubernetes backend (#4092)

According to [the documentation](https://woodpecker-ci.org/docs/administration/backends/kubernetes#images-from-private-registries), per-organization and per-pipeline registries are currently unsupported for the Kubernetes backend.

This patch implements this missing functionality by creating and deleting a matching secret for each pod with a matched registry, using the same name, labels, and annotations as the pod, and appending it to its `imagePullSecrets` list.

This patch adds tests for the new functionality, and has been manually end-to-end-tested in KinD by using a private image hosted in the matching gitea instance.

This will require updating the matching helm charts to add the create/delete permissions to the agent role, which **is already done**.

close  #2987

pipeline/backend/kubernetes/kubernetes.go

@@ -16,6 +16,7 @@ package kubernetes
 
 import (
 	"context"
+	std_errs "errors"
 	"fmt"
 	"io"
 	"maps"
@@ -225,6 +226,13 @@ func (e *kube) StartStep(ctx context.Context, step *types.Step, taskUUID string)
 		log.Error().Err(err).Msg("could not parse backend options")
 	}
 
+	if needsRegistrySecret(step) {
+		err = startRegistrySecret(ctx, e, step)
+		if err != nil {
+			return err
+		}
+	}
+
 	log.Trace().Str("taskUUID", taskUUID).Msgf("starting step: %s", step.Name)
 	_, err = startPod(ctx, e, step, options)
 	return err
@@ -382,9 +390,20 @@ func (e *kube) TailStep(ctx context.Context, step *types.Step, taskUUID string)
 }
 
 func (e *kube) DestroyStep(ctx context.Context, step *types.Step, taskUUID string) error {
+	var errs []error
 	log.Trace().Str("taskUUID", taskUUID).Msgf("Stopping step: %s", step.Name)
+	if needsRegistrySecret(step) {
+		err := stopRegistrySecret(ctx, e, step, defaultDeleteOptions)
+		if err != nil {
+			errs = append(errs, err)
+		}
+	}
+
 	err := stopPod(ctx, e, step, defaultDeleteOptions)
-	return err
+	if err != nil {
+		errs = append(errs, err)
+	}
+	return std_errs.Join(errs...)
 }
 
 // DestroyWorkflow destroys the pipeline environment.