zeek add support for iso8601 timestamps

[leehinman]

What does this PR do?

Adds support for ISO8601 timestamps in zeek

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).

How to test this PR locally

elastic-package test pipeline

Related issues

• Relates Add ISO8601 as supported timestamp type beats#25564

[elasticmachine]

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

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Build Cause: Pull request #1118 updated

• Start Time: 2021-06-17T13:47:20.755+0000

• Duration: 54 min 49 sec

• Commit: d0b414f

Test stats 🧪

Test	Results
Failed	0
Passed	191
Skipped	0
Total	191

Trends 🧪

[P1llus]

LGTM, hopefully the merge conflicts isn't too much