When making a custom field type, it must implement Indexable (even though the field content should not be searchable) or else the solr bundle will return exception:

OutOfBoundsException:
Field Type "mycustomfieldtype" is not indexable. Provide Ibexa\Contracts\Core\FieldType\Indexable implementation and register it with the "ibexa.field_type.indexable" tag.
 
  at vendor/ibexa/core/src/lib/Search/Common/FieldRegistry.php:39
  at Ibexa\Core\Search\Common\FieldRegistry->getType('ezuser')
     (vendor/ibexa/solr/src/lib/FieldMapper/ContentTranslationFieldMapper/BlockDocumentsContentFields.php:79)

Legacy search and Elastic search will not throw any errors.

It is a bit unclear to me if it is intended that all fieldtypes must implement Indexable or not.
pro:

• All core fieldtypes implement Indexable, also those that is not searchable

cons:

• Documentation doesn't say it is required unless you want data be searchable
• All other search engine adapters (legacy search and elastic) doesn't require it.

So either documentation should be updated or Solr bundle should behave as the other search engine adapters. This PR adds support in Solr for not having to implement Indexable