Embed static files (templates) into NIC Go binary

[jjngx]

At present templates are managed separately from the NIC Go binary. Embedding (reference) them into the binary would provide following benefits:

• TemplateExecutor would always have access to the original templates; no need to keep track of template's content (state) internally (inside the executor stuct) when users use custom templates (via ConfigMaps)
• no need to read content of files at startup as the files content is processed at compile time
• remove the need to manage static files while building Docker images
  for example:

https://github.com/nginxinc/kubernetes-ingress/blob/5108b30904954e1707c6bb48471d727cb161adca/build/Dockerfile#L666L670

https://github.com/nginxinc/kubernetes-ingress/blob/5108b30904954e1707c6bb48471d727cb161adca/build/Dockerfile#L714L718

https://github.com/nginxinc/kubernetes-ingress/blob/5108b30904954e1707c6bb48471d727cb161adca/build/Dockerfile#L763L766

• remove template files from Docker containers

nginx@nginx-ingress-68449896cb-4jjpp:/$ ls -l
total 59036
...
-rwxr-xr-x   1 nginx root 60321944 Oct 15 11:20 nginx-ingress
-rw-rw-r--   1 nginx root    14450 Oct  1 14:06 nginx-plus.ingress.tmpl
-rw-rw-r--   1 nginx root    12320 Oct  1 14:06 nginx-plus.tmpl
-rw-rw-r--   1 nginx root     2531 Oct  1 14:06 nginx-plus.transportserver.tmpl
-rw-rw-r--   1 nginx root    25478 Oct  1 14:06 nginx-plus.virtualserver.tmpl
...

Users could still use their custom templates when:

• using ConfigMaps
• specifying their template files via CLI arguments at NIC startup