feat: Support for CustomResource's Image Injection

**Kind**
This is a feature request.

**Background**
As of now, Skaffold only support image injection for Kubernetes kind such as `Deployment`.
I think it will be really valuable to support image injection for `CustomResourceDefinition`. Given a certain `CustomResource` (CR), eg:

```
apiVersion: example.com/v1
kind: MyCustomResource
spec:
template:
spec:
containers:
- name:
```

For each of CR's kind it will be possible for user to define where Skaffold will be able to do the image injection, in this case it'll be on the `IMAGENAME`.

I'm not sure on how this can be done, perhaps we can use the `patch` operation and we define the path where we should perform the image injection alongside the kind where this patch operation should be performed.