Pre-aggregated field mappers

[iverase]

Meta issue to track the development of new field types which consist in pre-aggregated data. Similar to the hash, geo or completion data types, these would expect input data to adhere to some kind of complex format. Two new mapping fields are targeted:

• Histograms: aggregation format for numeric fields.

• HLLplus: cardinality aggregation format for numeric / keyword fields.

Behaviour at indexing time

This complex data structures won't be indexed but stored as a binary doc values. Therefore this fields cannot be using in search (except for exists query) but they can be used for aggregations.

Aggregations

The new field should be able to be used with standard aggregations. In addition they should be able to work with a combination of standard(raw) and pre-aggregated fields. In particular:

• Histograms: They should be aggregated using percentiles and percentile_ranks aggregations.

• HLLplus: They should be aggregated using cardinality aggregation,

Relates to #33214

[elasticmachine]

Pinging @elastic/es-analytics-geo (:Analytics/Aggregations)