Allow to add custom fields configured via append_fields to be added to default_fields

[urso]

Predefined fields can be included/excluded to index.query.default_field in the template if include_in_default_field is configured. The same setting should be available for custom fields added via append_fields.

Elasticsearch indices.query.bool.max_clause_count limits the number of clauses (default 1024). For each field in default_fields a clause will be generated. If we hit a limit of 1024 entries in the template, we should also log a warning pointing users at the Elasticsearch docs 1

[urso]

Related: #14262
https://github.com/elastic/enhancements/issues/5922

[botelastic]

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