[crowdstrike] Add support to parse the custom notification format provided by Crowdstrike

[marc-gr]

What does this PR do?

Adds support for the custom format of the FDR SQS notifications.

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).

Closes #2193

[elasticmachine]

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Build stats

• Start Time: 2021-11-30T17:14:37.175+0000

• Duration: 16 min 36 sec

• Commit: 74d5204

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Test	Results
Failed	0
Passed	13
Skipped	0
Total	13

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[elasticmachine]

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

[andrewkroh]

LGTM