A validation webhook to prevent some EventBus spec to be updated

[whynowy]

Is your feature request related to a problem? Please describe.
EventBus is essential to the whole Argo Events, it's very critical to EventSource and Sensor services. Updating EventBus spec might cause unexpected behavior once the EventBus is created (e.g. updating auth strategy from none to token), this is something not we expected.

Describe the solution you'd like
A validation webhook to prevent this kind of action will be part of the installation.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

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[whynowy]

#1021