Adds enum certificate_source to pages_domains table
with default manually_uploaded

Mark certificates as 'gitlab_provided'
if the were obtained through Let's Encrypt

Mark certificates as 'user_provided' if they were uploaded through
controller or api

Only show private key in domain edit form if it is 'user_provided'

Only show LetsEncrypt option if is enabled by application settings
(and feature flag)

Refactor and fix some specs to match new logic

Don't show Let's Encrypt certificates as well

Save certificate validity time for pages domains on save
Fill validity time for existing pages domains in background migration

This reverts merge request !28743

Save certificate validity time for pages domains on save
Fill validity time for existing pages domains in background migration

Domain will be removed by verification worker after 1 week
of being disabled

Adds `# frozen_string_literal: true` to spec/models ruby files

Closes #28857

Remove superfluous lib: true, type: redis, service: true, models: true, services: true, no_db: true, api: true
Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Previously, `PagesDomain` would not allow a domain such as
123.example.com. With this change, this is now allowed, because
it is a perfectly valid domain.