Improve accessing unmapped via source only runtime fields

[martijnvg]

Source only runtime fields are runtime fields without scripts. The name of the runtime field implies which field from source is going to be loaded. For synthetic source, we significantly improve accessing unmapped fields using these types of runtime fields. This meta issue is around improving the performance of accessing source only runtime fields in the case source mode is synthetic.

Tasks:

• Generically improve accessing synthetic source in *FieldScript.Factory (Runtime fields: pass down runtime field name as source filter when source mode is synthetic #133897)
• Improve LongScriptFieldType block loader by using the FallbackSyntheticSourceBlockLoader instead of generic LongScriptBlockLoader. (Improve block loader for source only runtime fields of type long. #134117)
• Improve DoubleScriptFieldType block loader. (Improve block loader for source only runtime fields of type double. #134629)
• Improve KeywordScriptFieldType block loader. (Improve block loader for source only runtime fields of type keyword #135026)
• Improve DateScriptFieldType block loader. (Improve block loader for source only runtime date fields #135373)
• Improve IpScriptFieldType block loader. Improve block loader for source only runtime IP fields #135393
• Improve BooleanScriptFieldType block loader. (Improve block loader for source only runtime boolean fields #135318)
• Improve GeoShapeScriptFieldType block loader.
• Improve GeoPointScriptFieldType block loader.

[elasticsearchmachine]

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