[fortinet_fortigate] Fix syslog KV split to take into account quoted values.

[marc-gr]

What does this PR do?

Fix syslog KV split to take into account quoted values and be more defensive.

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.

[elasticmachine]

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

[elasticmachine]

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Build stats

• Start Time: 2023-05-22T08:31:51.177+0000

• Duration: 16 min 32 sec

Test stats 🧪

Test	Results
Failed	0
Passed	9
Skipped	0
Total	9

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (4/4)	💚
Classes	100.0% (4/4)	💚
Methods	100.0% (38/38)	💚
Lines	90.952% (1156/1271)	👎 -2.636
Conditionals	100.0% (0/0)	💚

[ShourieG]

LGTM

[elasticmachine]

Package fortinet_fortigate - 1.10.1 containing this change is available at https://epr.elastic.co/search?package=fortinet_fortigate