Add support for alias in Kibana index pattern generation

[ruflin]

Beats automatically generates the index pattern for Kibana based on the fields.yml. Currently all fields marked as type: alias are skipped.

The type that should be loaded into Kibana is the same as the original field. Unfortunately there is no type: alias concept in Kibana. This means we need to look up the generation of the type in our own fields based on the path.

This feature is needed to have the alias also listed in the fields when migration.enabled: true is used.

Note: It seems at the moment if there is 6.x and 7.x data with migration alias enabled things work as expected as the fields are already defined in the template and Kibana as mentioned above does not make a difference if it's an original field or not.

[ruflin]

Closing as this was introduced in #9805