feat: add headers support for remote Weaviate third-party API keys

[shubham3121]

This pull request adds support for specifying additional HTTP headers for third-party API keys (such as Cohere, OpenAI, HuggingFace) in the configuration and usage of remote Weaviate datasets. These headers can now be entered via the frontend and are passed through to backend API calls, enabling integrations with external vectorizers that require custom authentication.

Backend enhancements for third-party API integration

• Added an optional headers field to the RemoteWeaviateConfiguration model, allowing users to specify additional HTTP headers for third-party API keys.
• Updated the backend Weaviate client initialization in both the search and healthcheck methods to include the new headers field, ensuring these headers are sent with API requests. [1] [2]

Frontend support for custom headers

• Implemented a new "Additional Headers" section in ExperimentalRemoteWeaviateDatasetPage.vue where users can add, remove, and select common header keys for third-party API integrations. Header values are masked for privacy.
• Updated frontend form state and submission logic to handle the new headers field, including conversion between array/object formats and masking in the JSON preview. [1] [2] [3] [4] [5] [6] [7]

Minor backend improvement

• Improved metadata extraction logic in the backend to handle creation_time more robustly.