[Feature Request] Custom domains on OFC using IngressOperator

[Waterdrips]

There has been requests to allow custom domains in OFC (#520)

We now have FunctionIngress CRD provided by the IngressOperator provided as an option in the ofc-bootstrap installer.

We could read annotations like

com.openfaas.cloud.custom-domain: "my-fn.example.com"
com.openfaas.cloud.custom-domain.tls: true

and create the FunctionIngress CRD.

Provided a user points their domain at the cluster (A or CNAME? ) then they could get a LetsEncrypt TLS cert if they wanted.

Im suggesting:

• Default TLS to false (LE rate limit, people might not want to lock their accounts out)
• Only 1 custom domain per function

[Waterdrips]

cc @alexellis issue for discussions