CodeIgniter 4: Missing support for .env configuration

[francescobianco]

Labels: enhancement, config
Context: CodeIgniter 4 with .env-based configuration

Description:
The SDK does not detect environment variables set in .env. Developers must hardcode API keys in code, which violates best practices.

Expected:
Provide helper: Openapi::fromEnv('OPENAPI_KEY', 'OPENAPI_URL');

Impact:
Reduced security and configuration portability between environments.

[Seraphim200001]

@francescobianco I would like to work on this as my next task.

[francescobianco]

@Seraphim200001 Great starting point! The only thing to keep in mind is to open the pull request against this branch: https://github.com/openapi/openapi-php-sdk/tree/upkeep

[Seraphim200001]

@francescobianco I've finished this part, but I'll wait until my previous PR is merged to ensure this new one stays clean.

I implemented a system to support environment variables across different frameworks (CodeIgniter, Symfony, Laravel, etc.). I’ve added support for the following keys:

For Client.php:
OPEN_API_TOKEN
OPENAPI_BASE_URL

For OauthClient.php:
OPENAPI_OAUTH_USERNAME
OPENAPI_OAUTH_APIKEY
OPENAPI_OAUTH_TEST_URL
OPENAPI_OAUTH_URL

Please let me know if there are any other places where we might need environment variable support!