mimecast: fingerprint audit events based on their event ID

[efd6]

What does this PR do?

This fingerprints audit event documents based on their event ID to prevent duplicate document ingestion.

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.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes [Mimecast] Duplicate events #5010

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Build stats

• Start Time: 2023-01-16T23:22:14.737+0000

• Duration: 20 min 31 sec

Test stats 🧪

Test	Results
Failed	0
Passed	56
Skipped	0
Total	56

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (8/8)	💚
Files	100.0% (8/8)	💚
Classes	100.0% (8/8)	💚
Methods	92.308% (108/117)	❕
Lines	87.994% (1224/1391)	❕
Conditionals	100.0% (0/0)	💚

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

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Package mimecast - 1.6.0 containing this change is available at https://epr.elastic.co/search?package=mimecast