Make breadcrumb block cacheable

Problem/Motivation

From SystemBreadcrumbBlock:

<?php
  /**
   * {@inheritdoc}
   *
   * @todo Make cacheable as part of https://drupal.org/node/1805054
   */
  public function getCacheMaxAge() {
    return 0;
  }
?>

Proposed resolution

Once #1805054: Cache localized, access filtered, URL resolved, and rendered menu trees lands, doing this becomes possible.

But, it doesn't make sense to fix it as part of that issue, because it requires analyzing and potentially updating all BreadcrumbBuilderInterface implementations, of which there are about half a dozen. That interface may need to be changed as well.

Therefore, it should be done in a separate issue.

Update BreadcrumbBuilderInterface so it allows cacheability metadata to be returned for the built breadcrumbs, and analyze all existing BreadcrumbBuilderInterface implementations to have the correct cacheability.

Remaining tasks

None.

User interface changes

None.

API changes

1. \Drupal\Core\Breadcrumb\BreadcrumbBuilderInterface::build() needs to return a Breadcrumb object (or NULL?) that can encapsulate what it is returning in HEAD plus cacheability metadata
2. Added a url.path cache context
3. hook_breadcrumb_alter takes $breadcrumb object as a first parameter instead of an array.

Beta phase evaluation