Expand runtime_field data types to include ranges

Describe the feature:

runtime_fields currently don't support range, e.g. date ranges, as data types (docs). This could be useful for analytical use cases that rely on durations.

Describe a specific use case for the feature:

An index may hold two or more fields that mark the start and end of a period, e.g. a session. There is currently no easy way to visualize such date ranges within Kibana using runtime_fields for experimental purposes, instead an index mapping would have to be deployed grabbing data from said date fields.