Create a secret for the license, rather than using (only) a flag (for the operator)

Create a secret for the inlets-pro license, rather than using (only) a flag

## Expected Behaviour

The license should be read from a file as not to leak the value in `kubectl get deploy inlets-operator`

## Current Behaviour

The license is shown in the deployment and via helm install when it's passed as a flag.

## Possible Solution

Using a secret, like we do for the API access token would make sense.

A change in the arkade app for the inlets-operator would also be required.

This is where the license is being read as an arg:

https://github.com/inlets/inlets-operator/blob/master/main.go#L79

Here is an example of reading a file (name passed via flag):

https://github.com/inlets/inlets-operator/blob/master/main.go#L74

And here is the helm chart to update:

https://github.com/inlets/inlets-operator/blob/master/chart/inlets-operator/templates/deployment.yaml#L36

Add an `if` statement and attach a volume in the same way as we do for a secret when the file is given instead of a literal value.