Namespace-specific configurations

# Summary

Allow a way to set certain controller configurations to only act in certain namespaces.

# Motivation

A user may be running Argo on different namespaces with different requirements. The ability to specify different options per namespace would be helpful.

# Proposal

On the config map:

```yaml
data:
namespaceConfig:
- namespace: "argo"
configMapRef: ...
```

The config maps would be then strategically merged.
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**Message from the maintainers**:

If you wish to see this enhancement implemented please add a 👍 reaction to this issue! We often sort issues this way to know what to prioritize.

No files, tests, or specific entry points are identified. Start by locating the controller's ConfigMap parsing and configuration-merging entry points; done means defined controller options can be selected by namespace and merged according to documented rules.

Written by the indexing model from the issue text.