interface fields are missing at inventory-networks index root level

[AlexRuiz7]

Description

The interface fields are not being added to the index template for the inventory-networks index. These fields are expected at the root level of the documents, as depicted in the index model definition at ecs/docs.

Plan

• Add the missing fields at the expected level of the index template.
• Update event generators.
• Add new index template to the setup plugin.
• Check documentation and tests are up-to-date.