Add field format information to schema

[andrewkroh]

It would be nice to supply additional information about formatting that can be used as a hint to anything that renders that field. For example indicate that client.bytes is bytes and could be formatted as 10KB or 100GB. Or indicate that event.duration is a time value given as nanoseconds.

- name: client.bytes
  format: bytes

- name: event.duration
  format: duration
  input_format: nanoseconds

Relates: elastic/beats#10184

[webmat]

Yes, agreed.

Pretty sure we can include in GA too. It's not a breaking change, just an enhancement.

[ruflin]

++ on adding it now and backport.

[jamiehynds]

Closing this as was added to ECS.