[cisco_ftd]: Set event.kind to alert when sha_disposition is malware or custom

[r00tu53r]

What does this PR do?

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.

How to test this PR locally

Pipeline tests:

elastic-package test pipeline -v -g

System Tests:

elastic-package test system -v -g

Related issues

• Closes Need a fix for Cisco Firepower malware event that should not be an alert #1556

[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

• Start Time: 2022-04-11T10:44:30.479+0000

• Duration: 13 min 10 sec

Test stats 🧪

Test	Results
Failed	0
Passed	17
Skipped	0
Total	17

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.