[EPM] log duplicate fields in kibana when creating index patterns

[neptunian]

As part of #56306, figure out how to log when there is a duplicate field with different types, that we need to save to the same index pattern. Currently handling of duplicate fields goes like this:

• If more than one field exists with the same name and type, merge the fields
• The merge works so that new field properties are added but will not overwrite the existing field's property if it already exists
• if the duplicate fields have different types, ignore this duplicate field, a merge is not possible, and continue.

[elasticmachine]

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