Continue on specific exit codes

# Summary

Support configuring specific exit codes with `continueOn`.

Could look something like:

```yaml
apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
generateName: exit-code-output-variable-
spec:
entrypoint: exit-code-output-variable
templates:
- name: exit-code-output-variable
steps:
- - name: failing-container
template: failing-container
continueOn:
exitCode: 3
- - name: echo-container
template: echo-container
arguments:
parameters:
- name: exitCode
value: "{{steps.failing-container.exitCode}}"

- name: failing-container
container:
image: alpine:3.6
command: [sh, -c]
args: ["exit 123"]

- name: echo-container
inputs:
parameters:
- name: exitCode
container:
image: alpine:3.6
command: [sh, -c]
args: ["echo \"Exit code was: {{inputs.parameters.exitCode}}\""]
```

# Use Cases

I have a container that can return a custom exit code. I would only like the workflow to continue if the exit code matches that, I don't want it to continue for any other failure.

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The issue provides a Workflow YAML example but names no repository files, tests, or entry points. Start by tracing the existing `continueOn` handling and exit-code propagation; done means workflows continue only for the configured exit code and preserve the exit code for later parameter use.

Written by the indexing model from the issue text.