[ti_misp] Fix parsing of threat event publish_timestamp

[chrisberkhout]

What does this PR do?

This fixes parsing of the threat event publish_timestamp field by adding a date step in the ingest pipeline that interprets the incoming value as a second-resolution UNIX timestamp, rather than letting Kibana later misinterpret it as a millisecond-resolution timestamp. This matches what is done for the threat data stream to what was already done for the publish_timestamp field in the threat_attributes data stream.

I updated the corresponding pipeline tests.

I manually verified that that the values come out as expected in the Kibana UI.

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.
• I have verified that Kibana version constraints are current according to guidelines.

Related issues

• Closes [ti_misp] Incorrect misp.event.publish_date #6492

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

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• Commit: 3a0e0a3

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• Start Time: 2023-06-14T13:19:42.837+0000

• Duration: 15 min 34 sec

Test stats 🧪

Test	Results
Failed	0
Passed	15
Skipped	0
Total	15

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (2/2)	💚
Files	100.0% (2/2)	💚
Classes	100.0% (2/2)	💚
Methods	100.0% (30/30)	💚 27.273
Lines	86.536% (617/713)	👎 -13.464
Conditionals	100.0% (0/0)	💚

[taylor-swanson]

LGTM

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Package ti_misp - 1.15.4 containing this change is available at https://epr.elastic.co/search?package=ti_misp