www/oauth2-proxy: convert rc script to be instance-aware

Like the tomcat ports or openhab, make the rc script instance aware.
To use it:

• cp /usr/local/etc/oauth2-proxy.cfg.sample /usr/local/etc/oauth2-proxy-myapp.cfg
• vi /usr/local/etc/oauth2-proxy-myapp.cfg
• ln -s oauth2_proxy /usr/local/etc/rc.d/oauth2_proxy_myapp
• sysctl oauth2_proxy_myapp_enable=YES
• service oauth2_proxy_myapp start

Approved by: maintainer timeout (3 weeks)
PR: 278325