Optimize block loader for `geo_point` in case of synthetic _source

[lkts]

Currently doc_values are used in block loader only when FieldExtractPreference is DOC_VALUES https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/index/mapper/GeoPointFieldMapper.java#L536. With synthetic source, block loader from source will be used. However if doc_values are present, synthetic source will be built from those same doc_values. We should directly use doc_values in this case instead of the _source indirection.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[elasticsearchmachine]

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

[hye-on]

Hi @lkts Can I take this on? :)

[lkts]

@hye-on you are welcome to