External-dns: Add source implementation for Heptio Contour IngressRoute

[jonasrmichel]

The Heptio Contour ingress controller supports both the standard Ingress type and a custom IngressRoute type.

Work is underway to add Status support for the Ingress type, which will enable Contour users to leverage External DNS through the standard Ingress source. However, Contour users who opt to use or whose use case requires use of the custom IngressRoute type are left without any ability to leverage External DNS.

To support these users, a dedicated source implementation for the Heptio Contour IngressRoute type is needed.

[jonasrmichel]

Closing this issue as it was addressed by #1084.

[seh]

Is there any plan to incorporate support for Contour's _HTTPProxy_ CRD?