Allow string for "groups" in OIDC claims

# Summary

Currently Argo Workflows only accepts array of strings for `groups` field from OIDC provider. https://github.com/argoproj/argo-workflows/blob/95cd467c7e75c38548bf42a5f3c940ac61568e4b/server/auth/types/claims.go#L13

So, the OIDC provider need to provide groups like this:
```json
"groups": ["user"]
```

But I'd like Argo Workflows accept string value as well.

```json
"groups": "user"
```

# Use Cases

Our OIDC provider returns string value as `groups` if the user only belong to one group. As I searched I could not find any standard schema definition of claims, so string `groups` is still possible. Also, I found similar issue in kubernetes repository which was accepted. https://github.com/kubernetes/kubernetes/pull/33332

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**Message from the maintainers**:

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Start in server/auth/types/claims.go at the OIDC groups claim definition and review how claims are decoded. Verify the behavior with the repository's existing authentication tests or test entry points, if available; done means both a single string and an array of strings are accepted without breaking the existing array form.

Written by the indexing model from the issue text.