[okta] Make event.original optional

[marc-gr]

What does this PR do?

Makes event.original optional in the same way as the rest of packages.

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).

Related issues

• Relates event.original optionality across all packages #777
• Relates Move Security integrations to GA #961

Screenshots

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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Build stats

• Build Cause: Branch indexing

• Start Time: 2021-05-26T22:28:35.568+0000

• Duration: 11 min 40 sec

• Commit: 30d0d05

Test stats 🧪

Test	Results
Failed	0
Passed	12
Skipped	0
Total	12

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[P1llus]

LGTM, added a small comment, and if you could bump ECS version as well it would be great.