Unable to create custom domain without a certificate via API
### Problem to solve
Unable to create custom domain without a certificate via API, due to the impossibility of disabling the `HTTPS-only` setting
### Further details
On the web UI of gitlab pages sections, you are capable of creating a new custom domain without a certificate, disabling the `Force domains with SSL certificates to use HTTPS` checkbox. In this way, later you can specify a custom domain name without the need of adding a certificate.

BUT, this is not possible using the API. If you call the api without specify a certificate you will get a:
{"message":{"certificate":["must be present if HTTPS-only is enabled"],"key":["must be present if HTTPS-only is enabled"]}}
### Proposal
Add someway to change this `HTPPS-only` setting via API. Or add another parameter on the create new pages domain, to disable at the same time that custom domain is created
### What does success look like, and how can we measure that?
You would be able to make a call like this (I have removed the '`--form "certificate=$CERT_PEM" --form "key=$KEY_PEM"`' part:
`curl --request POST --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" --form "domain=www.domain.example" https://gitlab.example.com/api/v4/projects/5/pages/domains`
and obtain a success instead of a 400 error
issue