add new proposal for external resource plugin

docs/proposals/external-resource-plugin.md

@@ -0,0 +1,135 @@
+---
+title: Statefulset support
+authors:
+  - '@aburan28'
+sponsors:
+  - '@zaller'
+creation-date: 2025-01-18
+---
+
+## Summary
+
+Currently Argo Rollouts only supports the `Deployment` workload type. There are a variety of other workloads that others would like to perform canary/blue-green upgrades on such as statefulsets or daemonsets. 
+
+There are a couple of options for implementation of these workloads but overall 
+
+
+## Motivation  
+In order to support other workloads in Rollouts besides Deployments a Rollout resource plugin would be ideal. 
+
+
+
+## User Stories
+As a developer, it would be great to define blue-green/canary rollouts for statefulsets/daemonsets. 
+
+### Goals
+
+- Allow open source community to implement rollouts for external resource types 
+
+
+
+
+
+
+
+### Proposal
+There exist several other plugin types within the Argo Rollouts codebase including plugins such as `stepPlugins`, `metricsPlugins`, and `trafficRouting` plugins. 
+
+This implementation would follow in those plugin footsteps and take the same approach. 
+
+
+
+```yaml
+apiVersion: argoproj.io/v1alpha1
+kind: Rollout
+metadata:
+  name: statefulset
+  labels:
+    blah: blah
+  annotations:
+    blah: balh
+spec:
+  resourcePlugin:
+    name: statefulset-plugin
+
+  template:
+    metadata:
+    spec:
+
+
+
+```
+
+
+```yaml
+
+status:
+  canary:
+    resourcePluginStatus:
+      - index: 1
+        name: statefulset
+        message: Call completed with status code 302
+        phase: Successful
+        startedAt: '2024-02-15T20:05:40Z'
+        finishedAt: '2024-02-15T20:21:40Z'
+        status: {}
+```
+
+
+
+#### Resource Plugin Interface
+
+```go
+type Phase string
+
+const (
+	PhaseRunning      Phase = "Running"
+	PhaseSuccessful   Phase = "Successful"
+	PhaseFailed       Phase = "Failed"
+	PhaseError        Phase = "Error"
+)
+
+type ResourceContext struct {
+	PluginName   string
+	Config       map[string]interface{}
+	Status       map[string]interface{}
+}
+
+type ResourceStatus struct {
+  Index         int
+  Name          string
+  Phase         Phase
+  Message       string
+  StartedAt     Time
+  FinishedAt    Time
+  Status        map[string]interface{}
+}
+
+type ResourceResult struct {
+  Phase         Phase
+  Message       string
+  RequeueAfter  Duration
+  Status        map[string]interface{}
+}
+
+type ResourcePlugin interface {
+  Init() error
+  Run(rollout, resourceContext) (ResourceResult, error)
+  Terminate(rollout, resourceContext) (ResourceResult, error)
+  Abort(rollout, resourceContext) (ResourceResult, error)
+}
+
+```
+
+
+
+
+
+### Alternatives considered
+
+#### 1. support other resources in-tree
+Support additional workloads within the existing codebase. This would require signficant refactoring of the existing codebase each time a new resource is added. 
+
+
+#### 2. Rollout Plugin controller 
+Create a dedicated rollout plugin controller with a corresponding `RolloutPlugin` CRD.