Add invoice notification and SDK middleware features

[Eromosele0110]

Closes #137
Closes #138
Closes #139
Closes #140

Summary:

• Adds invoice notification template rendering with built-in templates and custom interpolation.
• Adds latency-aware LoadBalancer with health tracking and re-probe support.
• Adds Mermaid invoice payment flow diagram generation.
• Adds optional compression middleware helpers and client config support.
• Adds focused tests for all four features.
• Updates .gitignore for generated test/snapshot artifacts.

Note:
Local tests could not be run in this environment because node/npm were unavailable.

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