Create ADR for Istio egressGateway component (kyma-project#1091)

* Create ADR for Istio egress-gateway component

* Update 0009-egress-gateway.md

* Update docs/contributor/adr/0009-egress-gateway.md

Co-authored-by: Marek Kołodziejczak <[email protected]>

* Apply suggestions from code review

Co-authored-by: Natalia Sitko <[email protected]>

---------

Co-authored-by: Marek Kołodziejczak <[email protected]>
Co-authored-by: Natalia Sitko <[email protected]>

docs/contributor/adr/0009-egress-gateway.md

@@ -0,0 +1,99 @@
+# Support for **egressGateway** Configuration
+
+## Status
+
+Accepted
+
+## Context
+
+To support cluster configurations that require control of outbound traffic
+besides routing of incoming traffic, the Istio module needs to support `egressGateway`.
+This Istio component allows for intercepting traffic coming from in-mesh Pods to 
+targets outside of the cluster scope. This allows users to perform tasks that include
+monitoring and securing access to outbound resources.
+
+## Decision
+
+We will introduce configuration for `egressGateway` Istio component in the
+Istio CustomResource (CR). The API will allow to conditionally enable
+the `egressGateway` as well as configuration for [KubernetesResourcesSpec(k8s)](https://istio.io/latest/docs/reference/config/istio.operator.v1alpha1/#KubernetesResourcesSpec).
+
+## Consequences
+
+The Istio CR API will be extended with configuration related to `egressGateway`. The `components` section will now include an additional `egressGateway` field containing a boolean `enabled` flag as well as configuration for `KubernetesResourcesSpec(k8s)`. The `k8s` configuration will have the exact same structure as for `ingressGateway`.
+
+This results in the folowing Go structure:
+
+```go
+// EgressGateway defines configuration for Istio egressGateway
+type EgressGateway struct {
+	// +kubebuilder:validation:Optional
+	K8s *KubernetesResourcesConfig `json:"k8s"`
+        // +kubebuilder:validation:Optional
+        Enabled *bool
+}
+```
+
+The `KubernetesResourcesConfig` struct is [already defined](https://github.com/kyma-project/istio/blob/04890425c106ffd564d4c209994f99b4e692f9ec/api/v1alpha2/istio_structs.go#L37) in the Istio controller.
+
+An example Istio CR could look as follows:
+
+```yaml
+apiVersion: operator.kyma-project.io/v1alpha2
+kind: Istio
+metadata:
+  name: default
+  namespace: kyma-system
+spec:
+  components:
+    egressGateway:
+      enabled: true
+      k8s:
+        hpaSpec:
+          maxReplicas: 10
+          minReplicas: 3
+```
+
+## Default Values
+
+By default, the `egressGateway` component will be disabled.
+
+The `egressGateway` component must have default values set for when the user does not set up `k8s` values. Since the component will most likely have a high load, the best course of action would be to propose the exact values after executing performance tests and ensure that the availability does not drop when the component is in place.
+
+The baseline values could be the same as we use for Istio Ingress Gateway:
+
+```yaml
+# Full installation
+hpaSpec:
+  maxReplicas: 10
+  metrics:
+  - resource:
+      name: cpu
+      target:
+        type: Utilization
+        averageUtilization: 80
+    type: Resource
+  minReplicas: 3
+resources:
+  limits:
+    cpu: 2000m
+    memory: 1024Mi
+  requests:
+    cpu: 100m
+    memory: 128Mi
+```
+
+```yaml
+# Light installation
+hpaSpec:
+  maxReplicas: 1
+  minReplicas: 1
+resources:
+  limits:
+    cpu: 1000m
+    memory: 1024Mi
+  requests:
+    cpu: 10m
+    memory: 32Mi
+```
+