[panw] Preserve original event for syslog messages

[taylor-swanson]

What does this PR do?

• Add processor to preserve original event message when using tcp/udp
  with syslog in agent config

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

cd packages/panw && elastic-package test 

Related issues

• Closes [panw] Preserve original event doesn't save original event with new syslog processor #3774

[elasticmachine]

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Build stats

• Start Time: 2022-07-22T18:33:23.164+0000

• Duration: 21 min 22 sec

Test stats 🧪

Test	Results
Failed	0
Passed	51
Skipped	0
Total	51

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (15/15)	💚 2.941
Classes	100.0% (15/15)	💚 2.941
Methods	98.529% (67/68)	👍 8.798
Lines	94.93% (3183/3353)	👍 4.347
Conditionals	100.0% (0/0)	💚

[elasticmachine]

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

[efd6]

The behaviour here is subtle enough to warrant including the reason for it in the commit message. The explanation in the issue does a good job of it, so it could be copied in at squash.