chore: support request headers template

[louis-jan]

Describe Your Changes

This PR added support for custom request headers, such as anthropic requires version header to work. Deprecated API Key template since it's a part of request headers template already.

Self Checklist

This pull request includes changes to update the handling of API key templates to header templates, and to improve the filtering logic in the ModelDropdown component. The most important changes include renaming api_key_template to header_template across various files and modifying the ModelDropdown component to handle potential undefined values.

Updates to API key templates:

• core/src/types/engine/index.ts: Renamed api_key_template to header_template in the EngineMetadata type.
• web/screens/Settings/Engines/ModalAddRemoteEngine.tsx: Updated all references from apiKeyTemplate to headerTemplate and adjusted the corresponding form fields and error handling. [1] [2] [3] [4]
• web/screens/Settings/Engines/RemoteEngineSettings.tsx: Changed the label and input for API Key Template to Request Headers Template and updated the input type from Input to TextArea.

Improvements to filtering logic:

• web/containers/ModelDropdown/index.tsx: Modified the filtering logic to handle potential undefined values in the engineList by using optional chaining.

[github-actions]

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