Add event fields to log data streams

[fearful-symmetry]

What does this PR do?

This adds a variety of event fields to data streams where they were missing before. This fixes an issue where event.code was erroneously reported as long.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.

How to test this PR locally

• Pull down and build
• Run auth, system, or syslog, check to see if event.code exists, and that the mapping is correct.

[elasticmachine]

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Build stats

• Build Cause: Pull request #932 updated

• Start Time: 2021-04-26T16:52:11.502+0000

• Duration: 13 min 46 sec

• Commit: 03d8d77

Test stats 🧪

Test	Results
Failed	0
Passed	188
Skipped	0
Total	188

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