Support the `value` parameter for `constant_keyword` fields

[ebeahan]

The constant_keyword data type accepts the value parameter. On a constant_keyword field if the value parameter is specified, that value is used for all documents in the index. Otherwise, it is set based on the first document that gets indexed.

If a user wishes to use the ECS tooling to manage their ES index templates, they may also wish to control the value of value via their custom field definitions.

Example definition:

  - name: acme
    title: acme
    group: 2
    short: Fields describing acme-related needs.
    description: >
      Acme-related needs
    fields:
      - name: stream
        description: stream
        level: extended
        type: constant_keyword
        value: widgets