Add email notifications for CSV requirements import

What does this MR do?

Related to #246857 (closed)

Follow up to !46361 (merged) where Issuable::ImportCsv::BaseService was added to import requirements from a CSV file.

This MR adds the functionality to notify users once a Requirements CVS import is completed. The email will include the number of imported requirements if successful or the errors if failed.

Screenshots (strongly suggested)

Success	Error in lines	Parse error

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #246857 (closed)