FR: Custom HTTP base path

[Knud3]

Add support for custom HTTP base path.

I tried Docker Hub release with nginx ingress and rewrite, but it failed partially.

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: nginx
  annotations:
    nginx.ingress.kubernetes.io/rewrite-target: /$2
spec:
  ingressClassName: nginx
  rules:
  - http:
      paths:
      - path: /carbon(/|$)(.*)
        pathType: ImplementationSpecific
        backend:
          service:
            name: carbon-relay-ng
            port:
              name: http

https://example.com/carbon