Add pattern for Segment API tokens

What does this MR do?

Add a pattern for Segment API tokens, based on communications from Segment. See issue and parent epic for details.

What are the relevant issue numbers?

Detect Segment API tokens in Secret Detection (gitlab-org/gitlab#380975 - closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer