Support for configmaps in `withItems` and `withParam` of loops

# Summary

What change needs making?

Add a way to reference values from configmaps in `withItems` / `withParam` in loops. Something like

```yaml
apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
generateName: loops-
spec:
entrypoint: loop-example
templates:
- name: loop-example
steps:
- - name: print-message
template: whalesay
arguments:
parameters:
- name: message
value: "{{item}}"
withItems: # invoke whalesay once for each item in parallel
configMapKeyRef:
name: my-config
key: tables

- name: whalesay
inputs:
parameters:
- name: message
container:
image: docker/whalesay:latest
command: [cowsay]
args: ["{{inputs.parameters.message}}"]
```

```yaml
apiVersion: v1
kind: ConfigMap
metadata:
name: my-config
data:
tables:
- table1
- table2
- table3
```

# Use Cases

When would you use this?

My usecase is to reuse an existing configmap used by other deployments and I can easily change the configuration across my workflows and deployments.

---

**Message from the maintainers**:

Impacted by this bug? Give it a 👍. We prioritise the issues with the most 👍.

Start by locating the workflow loop handling for withItems and withParam, then trace how Kubernetes ConfigMap references are resolved elsewhere in the project. Confirm the expected behavior using the YAML examples: values from the named ConfigMap key should expand into loop items for workflow steps, while existing loop forms continue to work.

Written by the indexing model from the issue text.