Expose webhook data for User via its own data-builder class

What does this MR do?

Related to: #290290 (closed)

This refactor helps to get all event data required for firing User related webhooks from its own data-builder class, namely Gitlab::HookData::User so that this information is not private to SystemHooksService anymore.

No changelog added as this is only a refactor.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is only a refactor.
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
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Related to #290290 (closed)