Revert MTN MoMo SDK to original product-separated architecture

[nutcas3]

This change reverts the problematic pkg/api package structure that broke the fundamental design principles of the MTN MoMo SDK.

Problem:

• Introduced single-client approach with one subscription key for all products
• Broke MTN MoMo's API product separation (Collection, Disbursement, Remittance)
• Required users to subscribe to all products even when only using one
• Violated the flexibility and transparency of the real MTN MoMo API

Solution:

• Removed pkg/api package entirely
• Restored original momo.go implementation with product separation
• Maintained flexible subscription key configuration
• Preserved API structure alignment with MTN MoMo's product categories

Benefits Restored:

• Users only need subscription keys for products they actually use
• Cost efficiency by avoiding unused product subscriptions
• API transparency matching MTN MoMo's actual structure
• Proper error handling for unauthorized product access
  fixes Bug in the momo sdk package #28
  Changes:
• Delete: momo/pkg/ directory and all its contents
• Restore: Original momo.go with ClientConfig and product-specific services
• Update: README.md to reflect correct usage patterns
• Verify: All tests pass and SDK functionality maintained

This ensures the SDK maintains the flexibility and transparency that MTN MoMo API provides while supporting all current and future API methods.