[v2.9] Virtual Resource filters (#288)

Adds logic which adds virtual fields resources. This allows these fields
to be sorted/filtered on when the SQL cache is enabled. Id and
metadata.state.name were added as the first two fields.

Co-authored-by: Michael Bolot <michael.bolot@suse.com>

pkg/resources/virtual/virtual.go

@@ -0,0 +1,28 @@
+// Package virtual provides functions/resources to define virtual fields (fields which don't exist in k8s
+// but should be visible in the API) on resources
+package virtual
+
+import (
+	"github.com/rancher/steve/pkg/resources/virtual/common"
+	"k8s.io/apimachinery/pkg/runtime/schema"
+	"k8s.io/client-go/tools/cache"
+)
+
+// TransformBuilder builds transform functions for specified GVKs through GetTransformFunc
+type TransformBuilder struct {
+	defaultFields *common.DefaultFields
+}
+
+// NewTransformBuilder returns a TransformBuilder using the given summary cache
+func NewTransformBuilder(cache common.SummaryCache) *TransformBuilder {
+	return &TransformBuilder{
+		&common.DefaultFields{
+			Cache: cache,
+		},
+	}
+}
+
+// GetTransformFunc retrieves a TransformFunc for a given GVK. Currently only returns a transformFunc for defaultFields
+func (t *TransformBuilder) GetTransformFunc(_ schema.GroupVersionKind) cache.TransformFunc {
+	return t.defaultFields.GetTransform()
+}