[EPM] Implement correct handling of array field type in mappings

[skh]

The current handling of fields of type array in the generation of index template mappings is incomplete.

This task is to look at the current behavior of index template generation with regard to array fields in libbeats and implement it for mapping generation in ingest_manager/server/services/epm/elasticsearch/template/.

In particular, when no object_type is present the field shouldn't be added at all (see https://github.com/elastic/beats/blob/master/libbeat/template/processor.go#L123-L124 and https://github.com/elastic/beats/blob/master/libbeat/template/processor_test.go#L101-L107).

Also, add unit tests for the new behavior.

[elasticmachine]

Pinging @elastic/ingest-management (Feature:EPM)

[skh]

Fixed in #60266 after all.