Support Config Generation

[pleshakov]

This PR adds a new component - Generator - for generating NGINX
config.

UPDATE: example log output:

{"level":"info","ts":1647553393.865566,"logger":"eventLoop","msg":"Processing a change","host":"cafe.example.com"}
{Op:1 Host:{Value:cafe.example.com PathRouteGroups:[{Path:/ Routes:[{MatchIdx:0 RuleIdx:0 Source:0xc0002e2480}]} {Path:/coffee Routes:[{MatchIdx:0 RuleIdx:0 Source:0xc00040c780}]} {Path:/tea Routes:[{MatchIdx:0 RuleIdx:0 Source:0xc00040d380}]}]}}
server {
        server_name cafe.example.com;


        location / {
                proxy_set_header Host $host;
                proxy_pass http://unix:/var/lib/nginx/nginx-502-server.sock;
        }

        location /coffee {
                proxy_set_header Host $host;
                proxy_pass http://10.96.114.34:80;
        }

        location /tea {
                proxy_set_header Host $host;
                proxy_pass http://10.96.239.145:80;
        }

}

[pleshakov]

example of reported warning:

{"level":"info","ts":1648080976.7375154,"logger":"eventLoop","msg":"got warning while generating config","kind":"HTTPRoute","namespace":"default","name":"cafe","warning":"service default/main cannot be resolved: service default/main doesn't exist"}