[AI-FSSDK] (DO NOT REVIEW) [FSSDK-12273] Add support for custom headers in polling datafile managers

[raju-opti]

Summary

This PR adds support for custom headers in polling datafile managers (PollingConfigManager and AuthDatafilePollingConfigManager). Users can now pass custom headers when initializing the SDK client instance, which will be included in HTTP requests to fetch the datafile.

Reference implementation: optimizely/javascript-sdk#1107

Changes

• Added custom_headers parameter to PollingConfigManager.__init__()
• Added custom_headers parameter to Optimizely.__init__() and passed to config managers
• Updated fetch_datafile() methods to merge custom headers with internal headers
• User-provided headers take precedence over SDK internal headers
• Added comprehensive test coverage for custom headers functionality

Usage Example

```python
from optimizely import optimizely

Initialize SDK with custom headers

client = optimizely.Optimizely(
sdk_key='your_sdk_key',
custom_headers={
'X-Custom-Header': 'custom_value',
'Authorization': 'Bearer custom_token' # Overrides internal auth if needed
}
)
```

Testing

Added test cases covering:

• Custom headers included in datafile fetch requests
• Custom headers override internal SDK headers (including `If-Modified-Since` and `Authorization`)
• Both `PollingConfigManager` and `AuthDatafilePollingConfigManager` support custom headers

Related

• Jira: FSSDK-12273
• Reference: [FSSDK-12026] add customHeaders option to polling config manager javascript-sdk#1107

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