Improve accessing unmapped fields with logsdb

[martijnvg]

Today many fields are mapped, but in the near future when most fields will be unmapped, reading/accessing unmapped fields will be more important. Initially unmapped fields will be stored via ignored source. The expectation is that read performance of ignored source is be slower compared to reading from doc values. However it can't be too slow.

Recent investigation shoed that accessing unmapped field via insist_* esql command is now way too slow: elastic/rally-tracks#801 (comment)

This meta issue is about improving accessing unmapped fields.

• Improve reading FallbackSyntheticSourceBlockLoader #130887
• Investigate using specialized field visitor when loading ignored source. #130886
• Use an unique stored field name for each ignored source entry #130919
• Keep track of which field names are in ignored source. #130931

[elasticsearchmachine]

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

[martijnvg]

The #130676 draft PR explores #130887 and #130886.
The #130898 draft PR kind of builds on top of #130676 and explores #130919.