Add VPN Regions support with List, GetByPage, and GetByID methods

[arslanbekov]

Description

• Implement VPN Regions functionality in cloudconnexa/vpn_regions.go
• Add List() method to retrieve all VPN regions
• Add GetByID() method to retrieve a specific VPN region
• Update README.md with VPN Regions usage example
• Add comprehensive end-to-end tests for VPN Regions methods

Fixes #16 (issue)

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

expected)

• Documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the documentation accordingly