refactor(syncer): refine transform function (#606)

## What type of PR is this?

/kind refactor

## What this PR does / why we need it:

The current transform function has two capabilities: trimming to save
informer memory and influencing data stored in ES. This PR splits these
two capabilities into two stages, making the function definition clearer
and having the following advantages:

1. The first stage uses jsonPath instead of go template, which is easier
for users to use and less prone to errors, and ensures that the data in
the informer is a subset of the original object.

2. The second stage can use workqueue to retry retryable errors that
occur during the transformation process.

## Which issue(s) this PR fixes:

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Fixes #

pkg/kubernetes/apis/search/register.go

@@ -54,6 +54,8 @@ func addKnownTypes(scheme *runtime.Scheme) error {
 		&SyncResourcesList{},
 		&TransformRule{},
 		&TransformRuleList{},
+		&TrimRule{},
+		&TrimRuleList{},
 	)
 	return nil
 }

pkg/kubernetes/apis/search/types.go

@@ -105,6 +105,46 @@ type ResourceSyncRule struct {
 
 	// TransformRefName is the name of the TransformRule
 	TransformRefName string
+
+	// Trim defines the trimming strategy for the resources of the current type.
+	Trim *TrimRuleSpec `json:"trim,omitempty"`
+
+	// TrimRefName is the name of the TrimRule.
+	TrimRefName string `json:"trimRefName,omitempty"`
+}
+
+// +genclient
+// +genclient:nonNamespaced
+// +k8s:deepcopy-gen:interfaces=k8s.io/apimachinery/pkg/runtime.Object
+
+// TrimRule defines the strategy of trimming k8s objects, which can save
+// informer memory by discarding redundant fields.
+type TrimRule struct {
+	metav1.TypeMeta   `json:",inline"`
+	metav1.ObjectMeta `json:"metadata,omitempty"`
+
+	Spec TrimRuleSpec `json:"spec,omitempty"`
+}
+
+type TrimRuleSpec struct {
+	// Retain specifies which fields should be retained after trimming.
+	Retain TrimRuleRetainFields `json:"retain,omitempty"`
+}
+
+type TrimRuleRetainFields struct {
+	// JSONPaths specifies the path of the field to be retained.
+	// For usage, please refer to https://kubernetes.io/docs/reference/kubectl/jsonpath/
+	JSONPaths []string `json:"jsonPaths,omitempty"`
+}
+
+// +k8s:deepcopy-gen:interfaces=k8s.io/apimachinery/pkg/runtime.Object
+
+type TrimRuleList struct {
+	metav1.TypeMeta `json:",inline"`
+
+	metav1.ListMeta `json:"metadata,omitempty"`
+
+	Items []TrimRule `json:"items"`
 }
 
 // +genclient

pkg/kubernetes/apis/search/v1beta1/register.go

@@ -55,6 +55,8 @@ func addKnownTypes(scheme *runtime.Scheme) error {
 		&SyncResourcesList{},
 		&TransformRule{},
 		&TransformRuleList{},
+		&TrimRule{},
+		&TrimRuleList{},
 	)
 	metav1.AddToGroupVersion(scheme, SchemeGroupVersion)
 	return nil

pkg/kubernetes/apis/search/v1beta1/types.go

@@ -123,6 +123,52 @@ type ResourceSyncRule struct {
 	// TransformRefName is the name of the TransformRule
 	// +optional
 	TransformRefName string `json:"transformRefName,omitempty"`
+
+	// Trim defines the trimming strategy for the resources of the current type.
+	// +optional
+	Trim *TrimRuleSpec `json:"trim,omitempty"`
+
+	// TrimRefName is the name of the TrimRule.
+	// +optional
+	TrimRefName string `json:"trimRefName,omitempty"`
+}
+
+// +genclient
+// +genclient:nonNamespaced
+// +k8s:deepcopy-gen:interfaces=k8s.io/apimachinery/pkg/runtime.Object
+
+// TrimRule defines the strategy of trimming k8s objects, which can save
+// informer memory by discarding redundant fields.
+type TrimRule struct {
+	metav1.TypeMeta   `json:",inline"`
+	metav1.ObjectMeta `json:"metadata,omitempty"` //nolint:tagliatelle
+
+	// +optional
+	Spec TrimRuleSpec `json:"spec,omitempty"`
+}
+
+type TrimRuleSpec struct {
+	// Retain specifies which fields should be retained after trimming.
+	// +optional
+	Retain TrimRuleRetainFields `json:"retain,omitempty"`
+}
+
+type TrimRuleRetainFields struct {
+	// JSONPaths specifies the path of the field to be retained.
+	// For usage, please refer to https://kubernetes.io/docs/reference/kubectl/jsonpath/
+	// +optional
+	JSONPaths []string `json:"jsonPaths,omitempty"`
+}
+
+// +k8s:deepcopy-gen:interfaces=k8s.io/apimachinery/pkg/runtime.Object
+
+type TrimRuleList struct {
+	metav1.TypeMeta `json:",inline"`
+
+	// +optional
+	metav1.ListMeta `json:"metadata,omitempty"` //nolint:tagliatelle
+
+	Items []TrimRule `json:"items"`
 }
 
 // +genclient