[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

Start by locating the SDK configuration entry point and existing programmatic configuration, then trace how endpoint, headers, and timeout values are currently supplied. Add environment-variable handling with precedence, validation, documentation examples, and tests covering valid and invalid values; done means all three variables work without overriding explicit configuration.