cisco_asa,cisco_ftd: retain source and destination user emails if present

[efd6]

Proposed commit message

See title.

Note that the suggested approach was not used in order to ensure that he workgroup domain prefix is removed from the email address.

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

• [ ]

How to test this PR locally

Related issues

• Closes [Cisco] Populating source.user.email field #8895

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cc @efd6

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

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Package cisco_asa - 2.30.0 containing this change is available at https://epr.elastic.co/search?package=cisco_asa

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Package cisco_ftd - 3.1.0 containing this change is available at https://epr.elastic.co/search?package=cisco_ftd