synthetic_source_keep: "arrays" overtriggers for fields that parse arrays

[lkts]

This can be reproduced by this test https://github.com/elastic/elasticsearch/blob/main/server/src/test/java/org/elasticsearch/index/mapper/blockloader/GeoPointFieldBlockLoaderTests.java#L77. geo_point fields have custom handling of arrays and therefore synthetic_source_keep: "arrays" should not apply to it (arrays have special meaning so we don't know how to exactly preserve them). However we put values of geo_point field in _ignored_source in some cases.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[lkts]

The logic is here https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/index/mapper/DocumentParser.java#L463. I think we assume that this code path only handles singular values but we actually end up here when parsing fields with custom array handling because of https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/index/mapper/DocumentParser.java#L627.

[jdcryans]

@parkertimmins welcome to the team, here's something to look at!