Add email SMTP envelope fields

[mjwolf]

Add fields to represent SMTP envelope values for "MAIL FROM" and "RCPT TO" values.

These values are from the SMTP transmission protocol/envelope, and may be different than what is present in email headers. This can be useful for detecting spoofing, or tracking the true recipients of email.

• Have you signed the contributor license agreement? y
• Have you followed the contributor guidelines? y
• For proposing substantial changes or additions to the schema, have you reviewed the RFC process? y
• If submitting code/script changes, have you verified all tests pass locally using make test? y
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes? y
• Is your pull request against main? Unless there is a good reason otherwise, we prefer pull requests against main and will backport as needed. y
• Have you added an entry to the CHANGELOG.next.md? y

Closes #2351

[github-actions]

Documentation changes preview: https://ecs_bk_2413.docs-preview.app.elstc.co/diff