Is the nat traversal /p2p connection. You can use tor hidden service, tuntox or reverse SSH for that. The first two work when both ends are behind NAT.
You connect using the NAT traversal solution of your choice to the raspberry pi, and you forward proxy port from raspberry to a local port. You configure your browser to use localhost as a proxy.
You will have 2 pieces of the puzle.
Is the proxy, you can use tinyproxy or ssh -D
Is the nat traversal /p2p connection. You can use tor hidden service, tuntox or reverse SSH for that. The first two work when both ends are behind NAT.
You connect using the NAT traversal solution of your choice to the raspberry pi, and you forward proxy port from raspberry to a local port. You configure your browser to use localhost as a proxy.