Expose the versioned pages site URL during the ci build

What does this MR do and why?

Make the CI_PAGES_URL variable always return the full deployment URL, including path_prefix

Currently the CI_PAGES_URL only returns the main pages deployment path. If the user leverages parallel deployments, it does not include the user-specified path_prefix. This MR moves the creation of said variable to models/build.rb where we have access to the job spec to be able to deduct the full deployment URL from (ahead of its deployment). We also leverage the Gitlab::Pages::UrlBuilder that generates the path for real after the pages deployment has been build so we have a SSOT for the deployment URL spec.

Introduce a new variable CI_PAGES_HOSTNAME with the full projects domain, not just the instance default

The existing CI_PAGES_DOMAIN variable always returned the instance's pages domain, not the project's domain. Seeing as this has been there for a while, I thought it safe to keep this behaviour as-is and instead introduce a new variable CI_PAGES_HOSTNAME that returns the full domain.

refactor the Gitlab::Pages::UrlBuilder

The above changes required major changes in the Gitlab::Pages::UrlBuilder, which had organically grown to include more and more functionalities and special use-cases. I took this as an opportunity to refactor the class to a more declarative instead of a functional style. this will allow easier maintenance for future changes such as what might result from #498446

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Related to #416493 (closed)