No way for modules to alter the token replacement data

Problem/Motivation

I have a custom set of routes that uses parameters to display data, but the data isn't an entity at all. I've written my own hook_metatags_alter() to load in the defaults for my routes, but the problem is I want to provide some custom data to the token replacement, but there's no way to alter it.

Proposed resolution

Allow hook_metatags_alter() to let modules alter or add token replacement data:

function mymodule_metatags_alter(array &$metatags, array &$context) {
  if (mypage_special_route()) {
    $context['data']['my-data'] = mypage_special_route_data();
  }
}

Remaining tasks

User interface changes

None

API changes

• Additional context available to alter in hook_metatags_alter().
• Method definition of \Drupal\metatag\MetatagManager::generateElements() and \Drupal\metatag\MetatagManager::generateRawElements() from taking an entity object, to taking an array of token data and a langcode. The preparation of the token data now happens upstream in metatag_get_tags_from_route(). Note that while both of these methods were public, they were not defined in MetatagManagerInterface. The generateRawElements() method even had an unused parameter that was removed.

Data model changes

None