feat: add retrying k8s client implementation (#79)

* add retrying k8s client implementation

* implement review feedback

* try approach without reflection

* add possibility to inject context for cancellation into GroupVersionKindForObject and IsObjectNamespaced

---------

Co-authored-by: Valentin Gerlach <valentin.gerlach@sap.com>

Author: Diaphteiros

docs/README.md

@@ -14,6 +14,7 @@
 - [Key-Value Pairs](libs/pairs.md)
 - [Readiness Checks](libs/readiness.md)
 - [Kubernetes Resource Management](libs/resource.md)
+- [Retrying k8s Operations](libs/retry.md)
 - [Kubernetes Resource Status Updating](libs/status.md)
 - [Testing](libs/testing.md)
 - [Thread Management](libs/threads.md)

docs/libs/retry.md

@@ -0,0 +1,26 @@
+# Retrying k8s Operations
+
+The `pkg/retry` package contains a `Client` that wraps a `client.Client` while implementing the interface itself and retries any failed (= the returned error is not `nil`) operation.
+Methods that don't return an error are simply forwarded to the internal client.
+
+In addition to the `client.Client` interface's methods, the `retry.Client` also has `CreateOrUpdate` and `CreateOrPatch` methods, which use the corresponding controller-runtime implementations internally.
+
+The default retry parameters are:
+- retry every 100 milliseconds
+- don't increase retry interval
+- no maximum number of attempts
+- timeout after 1 second
+
+The `retry.Client` struct has builder-style methods to configure the parameters:
+```golang
+retryingClient := retry.NewRetryingClient(myClient).
+  WithTimeout(10 * time.Second). // try for at max 10 seconds
+  WithInterval(500 * time.Millisecond). // try every 500 milliseconds, but ...
+  WithBackoffMultiplier(2.0) // ... double the interval after each retry
+```
+
+For convenience, the `clusters.Cluster` type can return a retrying client for its internal client:
+```golang
+// cluster is of type *clusters.Cluster
+err := cluster.Retry().WithMaxAttempts(3).Get(...)
+```

pkg/clusters/cluster.go

@@ -12,6 +12,7 @@ import (
 	"sigs.k8s.io/controller-runtime/pkg/cluster"
 
 	"github.com/openmcp-project/controller-utils/pkg/controller"
+	"github.com/openmcp-project/controller-utils/pkg/retry"
 )
 
 type Cluster struct {
@@ -238,6 +239,12 @@ func (c *Cluster) APIServerEndpoint() string {
 	return c.restCfg.Host
 }
 
+// Retry returns a retrying client for the cluster.
+// Returns nil if the client has not been initialized.
+func (c *Cluster) Retry() *retry.Client {
+	return retry.NewRetryingClient(c.Client())
+}
+
 /////////////////
 // Serializing //
 /////////////////

pkg/pairs/pairs_test.go

@@ -14,7 +14,7 @@ import (
 func TestConditions(t *testing.T) {
 	RegisterFailHandler(Fail)
 
-	RunSpecs(t, "ClusterAccess Test Suite")
+	RunSpecs(t, "Pairs Test Suite")
 }
 
 type comparableIntAlias int