NATS Event Source - Support NKEY Credentials

[cawfeecoder]

Is your feature request related to a problem? Please describe.
Currently, the NATS Event Source cannot connect to clusters that integrate with nats-account-server (to provision accounts that have particular permissions). I'd like to see the NATS Event Source Spec (and implementation) pick up the ability to pass a reference to a secret that contains a valid NATS .creds file

Describe the solution you'd like
I'd like to see the NATS Event Source expanded in order to accommodate clusters that accept NKEYs in the form of .creds to authenticate.

Describe alternatives you've considered
If this doesn't get fulfilled, I'll likely write a bridge from a auth enabled NATS cluster to send the message over HTTP POST to the Argo Events Webhook Event Source

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

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Message from the maintainers:

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[whynowy]

@nfrush - Would you like to do a testing? You need to use the latest installation manifests: https://raw.githubusercontent.com/argoproj/argo-events/master/manifests/install.yaml, and change the images to:

gcr.io/whynowy/sensor:2.096
gcr.io/whynowy/sensor-controller:2.096
gcr.io/whynowy/eventsource:2.096
gcr.io/whynowy/eventsource-controller:2.096
gcr.io/whynowy/eventbus-controller:2.096