DEMO: Replace SECURITY_CONTACTS file with security_contacts in OWNERS

[sfowl]

This is example PR showing the proposed changes to security contact information (see #56), in this example, the component is client-go.

1. The SECURITY_CONTACTS file has been removed
2. A new key has been added to the OWNERS file, security_contacts (with subkeys github, email and slack)
3. Users previously in SECURITY_CONTACTS that are not PSC members have been added to security_contacts
4. approvers have also been added to security_contacts
5. Email addresses from public github profiles have also been included for security_contacts, some minor obfuscation has been applied

If this example is works, I can proceed with opening similar PRs against the entire kubernetes organization.