feat: add OpenTelemetry support with default no-op adapter

[cedricgarcia-wttj]

This pull request introduces OpenTelemetry integration into the Chargebeex library, enabling users to implement custom span instrumentation for tracing and monitoring. It includes a default no-op implementation and allows configuration of custom adapters. Additionally, OpenTelemetry spans are integrated into the Chargebeex.Client module for HTTP request tracing.

OpenTelemetry Integration

• Documentation Update: Added a new section in the README.md explaining OpenTelemetry integration, including default behavior, custom implementation examples, configuration, and usage.
• Default Implementation: Implemented a no-op module Chargebeex.OpenTelemetry.Default that adheres to the OpenTelemetry behavior, serving as the default adapter.
• Behavior Definition: Defined the Chargebeex.OpenTelemetry.OpenTelemetryBehaviour module, specifying the required callbacks for custom OpenTelemetry adapters.

Client Module Enhancements

• Tracing Integration: Integrated OpenTelemetry span creation into the Chargebeex.Client module for get and post methods, enabling automatic tracing of HTTP requests.
• Adapter Configuration: Added a private function otel_adapter/0 in Chargebeex.Client to fetch the configured OpenTelemetry adapter, defaulting to Chargebeex.OpenTelemetry.Default.