[Change Proposal] Add the field format property

### Overview

Include the `format` property in the field spec: https://github.com/elastic/package-spec/blob/main/versions/1/data_stream/fields/fields.spec.yml

The `format` field specifies how Kibana should display the field if Kibana defaults are suboptimal.

Example definition: https://github.com/elastic/integrations/blob/main/packages/system/data_stream/memory/fields/fields.yml#L6

### Background

Both [ECS](https://github.com/elastic/ecs/blob/main/schemas/process.yml#L77) and [non-ECS](https://github.com/elastic/integrations/blob/main/packages/system/data_stream/memory/fields/fields.yml#L6) fields use the `format` attribute to change how the field should be displayed in Kibana. For example, a TCP/UDP port number should be displayed as `12345` as a `String` instead of `12,345` as a `Number` in the Kibana data view.

Relevant section from Beats dev guide: https://github.com/elastic/beats/blob/main/docs/devguide/newdashboards.asciidoc#generate-index-pattern