Add configuration file and runtime management of NGINX

[pleshakov]

Problem:
(1) Add/update/delete NGINX configuration files to/from the file system.
(2) Reload NGINX to apply changes in the configuration files.

Solution:
For (1): Add a new component - file.Manager - that allows writing and removing configuration files for a server.
The Manager writes server configuration files to /etc/nginx/conf.d folder. This folder is available to both gateway and NGINX containers as part of the shared volume /etc/nginx

For (2): Add a new component - runtime.Manager - that allows reload NGINX. The Manager reloads NGINX by sending a HUP signal to the NGINX master process.
Because the gateway process is running under user 1001 (rather than root), during the image build we add the CAP_KILL capability to the gateway binary.

Notes to the reviewers:

(a) both file.Manager and runtime.Manager use OS dependencies. Some unit tests were added, but the coverage is not full. If you think it makes sense to extend the coverage, please share some advice.

(b) With configuration file and runtime management implemented, we now are able to pass traffic through the NGINX!

cd examples/cafe-example

kubectl apply -f cafe.yaml

Wait until all pods are running. endpoints changes are not handled right now by the Gateway

kubectl get pods
NAME                      READY   STATUS    RESTARTS   AGE
coffee-6f4b79b975-llpcx   1/1     Running   0          7s
tea-6fb46d899f-cb4cs      1/1     Running   0          7s

kubectl apply -f cafe-routes.yaml

kubectl -n nginx-gateway port-forward <nginx-gateway-pod-name> 8080:80

curl localhost:8080/coffee -H "host: cafe.example.com"
Server address: 10.244.0.6:8080
Server name: coffee-6f4b79b975-llpcx
Date: 25/Mar/2022:21:26:19 +0000
URI: /coffee
Request ID: 0ffc7aeb436065df432170ce0cf4a932

curl localhost:8080/tea -H "host: cafe.example.com"
Server address: 10.244.0.7:8080
Server name: tea-6fb46d899f-cb4cs
Date: 25/Mar/2022:21:26:25 +0000
URI: /tea
Request ID: 2c4227432a5ee9f699e96f704183db8

[kate-osborn]

Looks good! Just a couple small comments/suggestions.

[kate-osborn]

🚀