feat: add $allowedHostnames for multiple domain support

[kenjis]

Description
Supersedes #6759
Fixes #5807
Related #6746

• add Config\App::$allowedHostnames
• add URI::$baseURL, URI::setBaseURL() and URI::getBaseURL()
• IncomingRequest::__construct() sets URI::$baseURL
  • use HTTP_HOST when it is an allowed hostname
• refactor

Changes:

• current_url(), base_url(), site_url() will return the URL with allowed HTTP_HOST
• previous_url() will return the site URL with allowed HTTP_HOST when cannot get it from session nor HTTP_REFERER

The following functions/methods are affected (There may be others):

• Using site_url()
  • form_open()
  • audio()
  • embed()
  • img()
  • link_tag()
  • object()
  • script_tag()
  • source()
  • video()
  • RedirectResponse::route()
  • RedirectResponse::to()
  • Toolbar::prepare()
  • Toolbar::respond()
  • anchor()
  • anchor_popup()
  • base_url()
  • previous_url()
  • url_to()
• Using base_url()
  • RedirectException in CodeIgnter.php L377
  • UserAgent::isReferral()
• Using current_url()
  • ChromeLoggerHandler::__construct()
  • CodeIgniter::storePreviousURL()
  • Pager::ensureGroup()
  • Toolbar::run()

Ref #4651

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[MGatner]

I think this is a better solution. Thank you for enumerating the impacted methods and components: that would be a helpful addition to the changelogs.

One thing I would like to note: for a while now I have been a proponent of a separate class for "internal URIs". Using one class for internal logic and hostname resolution and normal URL parsing & passing... it seems to be multiple responsibilities and prone to problems. Past conversations have concluded that maintaining multiple URI classes is overkill and confusing. This PR mixes concerns even more so I would like to bring this conversation back up again for v5.

[kenjis]

Added the docs.

[datamweb]

Excellent, this feature always felt empty.