Add dataset and module fields to postgresql package

[jsoriano]

What does this PR do?

• Add dataset and module fields to postgresql package.
• Add references to external ECS fields.

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.
• If I'm introducing a new feature, I have modified the Kibana version constraint in my package's manifest.yml file to point to the latest Elastic stack release (e.g. ^7.13.0).

Author's Checklist

• [ ]

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Build stats

• Build Cause: Branch indexing

• Start Time: 2021-06-29T22:28:20.883+0000

• Duration: 12 min 2 sec

• Commit: 4f7b0fd

Test stats 🧪

Test	Results
Failed	0
Passed	58
Skipped	0
Total	58

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[mtojek]

LGTM