Service export should handle securityContext field

### Feature request

With more Pod Security Standards enabled clusters we need to address the `securityContext` being present in PodSpec and Containers definitions. Since the two cluster with different PSS config might not be compatible for the same export.

I'd suggest that we remove it in default behavior and add a flag to include it optionally

### UI Example

```

kn service export --with-securityContext
or
kn service export --with-sc
```

/kind good-first-issue
/kind proposal

/cc @rhuss @mvinkler