[Sophos] Fix parse of syslog priority from UTM events

[chemamartinez]

Proposed commit message

This PR fixes the processing of UTM events when they contain the syslog priority in the header.

In addition, from that priority, the following fields are mapped:

• log.syslog.severity.code
• log.syslog.severity.name
• log.syslog.facility.code
• log.syslog.facility.name

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

• Added syslog priority fields to several test cases in pipeline tests

Related issues

• Closes [sophos][utm] parsing fails completely, grok error #8430

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

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• Start Time: 2023-11-14T17:21:28.313+0000

• Duration: 18 min 25 sec

Test stats 🧪

Test	Results
Failed	0
Passed	38
Skipped	0
Total	38

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (2/2)	💚
Files	100.0% (17/17)	💚 3.823
Classes	100.0% (17/17)	💚 3.823
Methods	100.0% (125/125)	💚 7.744
Lines	91.053% (2534/2783)	👍 2.428
Conditionals	100.0% (0/0)	💚

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Package sophos - 3.6.1 containing this change is available at https://epr.elastic.co/search?package=sophos