import-beats: create base fields

[mtojek]

Issue: elastic/package-registry#465 (comment)

import-beats: create base-fields.yml files for each dataset

Testing:

I used haproxy integration for development, but didn't pull it in for clarity.

$ PACKAGES=haproxy mage ImportBeats
$ find packages/haproxy | grep base-fields
packages/haproxy/dataset/info/fields/base-fields.yml
packages/haproxy/dataset/stat/fields/base-fields.yml
packages/haproxy/dataset/log/fields/base-fields.yml
$ cat packages/haproxy/dataset/info/fields/base-fields.yml
- name: stream.type
  type: constant_keyword
  description: Stream type.
- name: stream.dataset
  type: constant_keyword
  description: Stream dataset.
- name: stream.namespace
  type: constant_keyword
  description: Stream namespace.
- name: '@timestamp'
  type: date
  description: Event timestamp.

[elasticmachine]

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• Build Cause: [Pull request #55 opened]

• Start Time: 2020-06-04T12:11:53.225+0000

• Duration: 4 min 46 sec

[ruflin]

Change LGTM.

I'm torn if it should be the import script creating this or the script that releases the package. To be discussed later.