Workflows on server-mode shouldn't require "argo-workflows-webhook-clients" if not using specific webhooks

# Summary

Currently there's a requirement that, if you're running on server-mode you'll need to add the Secret `argo-workflows-webhook-clients` as per described here: https://argoproj.github.io/argo-workflows/webhooks/

Reading the GoLang code, I saw that secret only gets used if I have a `webhook` from Gitlab, Bitbucket and so forth, but not if I just want to use it as an API instead, sending events through Curl, for example.

My app is on server-mode because authentication will happen using SSO and Keycloak under the ingress level, so I won't need it at app level as much, but I ended up having to create this Secret with dummy values just to make the workflow be triggered correctly.

I think the requirement of that Secret should be removed for such cases, and only used if the Service Account is one of the types you guys check.

# Use Cases

- When creating event binding on a argo-workflows running in need server mode, without the request being from any of the specific webhook types (Gitlab, Bitbucket, etc).

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**Message from the maintainers**:

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Start by tracing the Go server-mode startup path and the handling of the argo-workflows-webhook-clients Secret when creating an event binding. Confirm the behavior for GitLab, Bitbucket, and other specific webhook types versus curl/API requests. Done means server-mode API event bindings work without dummy Secret values while supported webhooks retain their required configuration.

Written by the indexing model from the issue text.