RFC to create a lifecycle crd

Signed-off-by: Tom Kennedy <[email protected]>

rfcs/0000-lifecycle-crd.md

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+`[Readable](https://github.com/your-name-or-org/kpack/blob/<pr-branch-name>/rfcs/0000-<my-feature>.md)`
+
+**Problem:**
+
+The CNB lifecycle is provided to kpack as in image reference in a configMap similar to how the completion and prepare
+images are passed to the kpack controller. This means that every builder will always use the same exact lifecycle, with
+no ability to specify an alternate one that may support different buildpack api or platform api versions. Due to kpack
+support of windows, a custom lifecycle image with specific layers for the linux and windows binaries is required
+(with a label specifying the layer diff id for each os). Because of this, a cve present in the lifecycle that has been 
+patched upstream, requires any user to build the kpack specific lifecycle image themselves if the kpack provided one
+has not been bumped.
+
+**Outcome:**
+
+Users will be able to create a CR that references a lifecycle image. The file structure in the image must match 
+that of the [CNB Lifecycle image](https://hub.docker.com/r/buildpacksio/lifecycle/tags).
+
+```
+.
+└── cnb/
+    └── lifecycle/
+        ├── analyzer
+        ├── builder
+        ├── detector
+        ├── exporter
+        ├── launcher
+        ├── lifecycle.toml
+        └── restorer
+```
+
+They will be able to use this lifecycle CR in any builder. A lifecycle will be shipped out of the box in kpack and 
+users will not be required to provide a lifecycle reference in the builder if they do not wish to override the default.
+Users should see no difference in functionality compared to the existing kpack lifecycle if they do not choose to 
+provide their own lifecycle CR.
+
+Instead of creating custom multi-layer lifecycle images, users can create a lifecycle CR for their desired runtime (i.e. windows or arm64).
+During builder creation, kpack will validate that the referenced lifecycle CR has the same OS and architecture as the base image.
+
+**Actions to take:**
+
+A new ClusterLifecycle CRD will be created. It will be very similar in structure to the ClusterStack Resource. 
+We will create a cluster scoped version of this resource. In the future, we may decide to create a namespaced version
+as well, but that is not in scope for this RFC. The name ClusterLifecycle aligns with kpack's existing cluster scoped 
+resources.
+
+This is an example instance of the proposed CRD:
+```yaml
+apiVersion: kpack.io/v1alpha2
+kind: ClusterLifecycle
+metadata:
+  name: sample-cluster-lifecycle
+spec:
+  serviceAccountRef:
+    name: sample-sa
+    namespace: sample-namespace
+  image: buildpacksio/lifecycle@sha256:f4ce143ea6bbc6b5be5f4d151aba8214324adb93bbd7e3b1f43cd134ad450bf7
+```
+
+
+The existing lifecycle reconciler can be updated to reconcile this new CRD instead of ConfigMaps.
+
+A new field will be added to the ClusterBuilder/Builder resources allowing the user to provide a reference to a lifecycle CR.
+This field will be optional and will default to the highest semantic version that is supported by the buildpacks in the builder and the platform. 
+This is similar to how we determine the platform api to use for a build.
+```yaml
+apiVersion: kpack.io/v1alpha2
+kind: Builder
+metadata:
+  name: my-builder
+spec:
+  tag: gcr.io/sample/builder
+  serviceAccountName: default
+  stack:
+    name: stack
+    kind: ClusterStack
+  lifecycle:
+    name: my-lifecycle
+    kind: ClusterLifecycle
+  order:
+  - group:
+    - id: paketo-buildpacks/java
+```
+
+```yaml
+apiVersion: kpack.io/v1alpha2
+kind: ClusterBuilder
+metadata:
+  name: my-cluster-builder
+spec:
+  tag: gcr.io/sample/builder
+  stack:
+    name: bionic-stack
+    kind: ClusterStack
+  lifecycle:
+    name: my-lifecycle
+    kind: ClusterLifecycle
+  serviceAccountRef:
+    name: default
+    namespace: default
+  order:
+  - group:
+    - id: paketo-buildpacks/java
+```
+
+Creating a CRD for the lifecycle will allow us to do the following:
+- Provide status resource that shows more information about the lifecycle such as supported api versions
+- Allow users to specify a particular lifecycle for their builder, mimicking behavior that currently exists in the pack cli
+- Offer a clean shift to using the upstream lifecycle image if done in conjunction with deprecation of windows support
+
+**Complexity:**
+
+The complexity of this is not that high due to its similarity with the other kpack resources.
+
+**Prior Art:**
+
+- The pack cli allows a lifecycle image to be specified when creating a build
+- Adding new resources to the builder that get added to a layer has been done before in kpack
+
+**Alternatives:**
+
+We could keep the existing experience.
+
+**Risks:**
+Exposing the user to the lifecycle can result in some confusion, but this can be mitigated by not making it a required 
+field on the builder and continuing to ship a lifecycle out of the box.