[OFREP] Implement support for OFREP Server SDK configuration using environment variables

Overview

Implement support for OFREP SDK configuration using environment variables in the SDK.

Goal

Enable users of the OFREP to configure the client using standardized environment variables, improving flexibility and consistency across OpenFeature Protocol implementations.

Scope

Environment Variables to Support

The following environment variables should be implemented:

• OFREP_ENDPOINT - The OFREP server endpoint (e.g., http://localhost:2321)
• OFREP_HEADERS - Custom headers in comma-separated format (e.g., "Authentication=Bearer 123,Content-Type=json")
• OFREP_TIMEOUT_MS - Request timeout in milliseconds (e.g., 5000)

Implementation Tasks

• Add support for reading OFREP_ENDPOINT environment variable
• Add support for reading OFREP_HEADERS environment variable with proper parsing
• Add support for reading OFREP_TIMEOUT_MS environment variable
• Ensure programmatic configuration takes precedence over environment variables
• Add validation and error handling for invalid environment variable values
• Update SDK documentation with environment variable usage examples
• Add tests for environment variable configuration

首先定位 OFREP SDK 的配置入口及其现有的编程式配置，然后检查请求和超时设置的表示方式。实现对 OFREP_ENDPOINT、OFREP_HEADERS 和 OFREP_TIMEOUT_MS 的处理，并以编程式值为优先级，同时按照 issue 中的描述添加验证、测试和文档示例。