Describe the bug:

Ingress routes for challenges created with pathType: Exact with no option of configuring this. This is unsupported by GCE ingress (it only supports ImplementationSpecific), which means that starting v1.5.0, cert-manager doesn't work with the GCE ingress.

The offending line is: https://github.com/jetstack/cert-manager/blob/v1.5.0/pkg/issuer/acme/http/ingress.go#L361

Rolling back to v1.4.3 was the only solution for us as this is not configurable. Relevant events from the ingress controller:

  Normal   Sync       3m43s (x119 over 3d18h)  loadbalancer-controller  Scheduled for sync
  Warning  Translate  3m27s (x2 over 5m1s)     loadbalancer-controller  Translation failed: invalid ingress spec: only "ImplementationSpecific" path type is supported; only "ImplementationSpecific" path type is supported; only "ImplementationSpecific" path type is supported

Expected behaviour:
ImplementationSpecific should possibly be the default for certain ingress controller. Alternatively this option should be configurable and not hard coded.

Steps to reproduce the bug:

Use the HTTP01 acme solver with the GCE ingress. There is currently no way to make this work.

Anything else we need to know?:

Environment details::

• Kubernetes version: v1.20.8-gke.900
• Cloud-provider/provisioner: GCP/GKE
• cert-manager version: v1.5.0/v1.5.1
• Install method: static manifests

/kind bug