Okta: populate user.email when user name is an email address

[chemamartinez]

Proposed commit message

According to ECS documentation about user fields usage[1]:

- When a system uses an email address as the main identifier, populate
  both user.id and user.email with it.

This change copies the `user.name` field into the `user.email` field to align
with all other integrations where the main user identifier is an email address,
including Entity Analytics Okta.

This should help anyone who wants to correlate email addresses between integrations.

In this case, the user email is not dissected into `user.name@user.domail` as
for the Okta ecosystem, the user part of the email is meaningless by itself. 

[1] https://www.elastic.co/docs/reference/ecs/ecs-user-usage#ecs-user-identifiers

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.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

Related issues

• Relates https://github.com/elastic/enhancements/issues/24531

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: adeaa4c

cc @chemamartinez

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube

[elastic-vault-github-plugin-prod]

Package okta - 3.10.0 containing this change is available at https://epr.elastic.co/package/okta/3.10.0/