Allow runtime_fields to be defined in the index mapping

[sothawo]

runtime_fields can be defined in the index mapping (https://www.elastic.co/guide/en/elasticsearch/reference/7.12/runtime-mapping-fields.html) or on a query. We already support them in NativeSearchQuery as scripted field.

It should be possible to define them on the index mapping level.

[sothawo]

Clarifying: we have scripted fields but not runtime fields in the query. This will be done with #1971