Add User-Agent header to all REST API calls #16

Summary

Add a consistent User-Agent header to all outgoing REST API calls made by apiops-cli. This enables tracking of apiops-cli usage in APIM server telemetry.

Requirements

• Add a User-Agent string (e.g., apiops-cli/) to all HTTP requests
• Define the User-Agent value in a single, centralized location so that if it needs to change, only one place requires modification
• Include the tool version in the User-Agent string for version tracking

Motivation

Having a recognizable User-Agent allows APIM administrators to:

• Track usage of apiops-cli in server logs and telemetry
• Distinguish apiops-cli traffic from other API clients
• Monitor adoption and usage patterns

Implementation Notes

• Consider defining a constant or utility function that returns the User-Agent string
• All HTTP client code should reference this single source of truth