Let's Encrypt through the page domain API endpoint

Problem to solve

There doesn't seem to be a way to enable lets encrypt through the page domain API endpoint: https://docs.gitlab.com/ee/api/pages_domains.html#create-new-pages-domain

There was discussion under [this bug](#30147](#30147) about setting letsencrypt to always be on but this seems to be for the interface and not the endpoint.

Possibly a separate issue but I didn't see anywhere that indicates that the page domain endpoint returns a DNS TXT record token for domain validation in the endpoint response.

Intended users

This feature is purely for developers who will be interfacing with gitlab.