Synthetic source failure when `doc_values: false`

[salvatore-campagna]

Description

All fields which support doc values have them enabled by default. Anyway, if sorting and/or aggregation and/or accessing a field from a script is not required users can disable doc values by specifying doc_values: false in index mappings.

LogsDB uses synthetic source by default and, as a result, fields without doc values cannot be reconstructed. If a field has doc_value: false and store: false in LogsDB we have to ways to deal with it:

1. fallback to using _ignored_source so that we can reconstruct the document contents including such fields too.
2. throw an error and require mappings not including doc_values: false

[elasticsearchmachine]

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

[lkts]

This was implemented for number fields and keyword. Pretty much every other field that natively supports synthetic source won't work without doc values. This work is pending.