[Change Proposal] Support 'synthetic_source_keep' in field mapping attributes

[synthetic_source_keep](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-source-field.html#synthetic-source-keep) is a mapping attribute added to Elasticsearch 8.16 that controls the behavior of a field under sythentic_source and LogsDB.

Integrations need to be able to specify this attribute to optimize the behavior of certain array fields in order to minimize storage costs. For example, an integration that stores a list of IP address would set `synthetic_source_keep: none` when the value is treated as a unsorted set (ordering does not matter and duplicate values have no meaning). Then the storage of the field can be optimized by not storing the original value in the `_source`.