Ability to opt out a field from fallback synthetic source

[lkts]

With synthetic _source enabled, values of fields that are not indexed (e.g. using dynamic: false) are stored in a special stored field. This is needed to replicate the behaviour of stored _source and enable reindexing that changes the mapping to now index such a field.

With stored _source it is possible to use excludes field on _source mapper to exclude the field from being stored in _source stored field. With this configuration a field will be not represented on disk in any way. This scenario is currently not possible using synthetic _source because excludes field is not compatible with synthetic _source. So in such case field values will be stored via fallback synthetic source making such setup less efficient in terms of storage space.

This issue is about exposing configuration options to achieve the same result of zero bytes being stored for a field.

[elasticsearchmachine]

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