[cef] Add time zone config option

[andrewkroh]

What does this PR do?

Expose the timezone config option for the decode_cef processor. It is an IANA time zone or time offset
(e.g. +0200) to use when interpreting timestamps without a time zone in the CEF message.

Relates: elastic/beats#27727

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).

Related issues

• Relates [Filebeat] Add timezone config option to decode_cef and syslog input beats#27727

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

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Build stats

• Start Time: 2021-09-27T18:59:08.070+0000

• Duration: 12 min 1 sec

• Commit: ab57073

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Test	Results
Failed	0
Passed	84
Skipped	0
Total	84

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