Computed fields can now bubble cache metadata in rendered output via their item lists

If a computed field is rendered using a formatter, the computed field item list class can now implement Drupal\Core\Cache\CacheableDependencyInterface in order to have it's cacheability metadata bubble to the rendered output.

Notes:

• The logic for this is added to Drupal\Core\Field\FormatterBase, so formatters not extending that base class may need to re-implement this logic
• This does not effect computed fields that are output via JSON:API (for now, see #3252278: Complete JSON:API's handling of field (including computed & empty fields) cache metadata)
• Theoretically the added logic applies to the field item list classes of non-computed fields, as well, but it generally does not make sense to apply cacheable metadata to non-computed fields