fix: get models for anthropic and cohere

[vansangpfiev]

Describe Your Changes

This pull request includes changes to the remote-engine component in the engine/extensions directory to improve the handling of API key headers and remove specific engine-related constants and logic. The most important changes include refactoring header handling, removing special handling for the kAnthropicEngine, and cleaning up unused constants.

Improvements to header handling:

• engine/extensions/remote-engine/remote_engine.cc: Refactored the MakeGetModelsRequest method to use a more flexible approach for replacing header placeholders using an unordered map and a loop to append headers.

Codebase simplification:

• engine/extensions/remote-engine/remote_engine.cc: Removed the special handling for the kAnthropicEngine in the GetRemoteModels method, simplifying the code path for empty URLs.
• engine/utils/engine_constants.h: Removed unused constants kOpenAiEngine and kAnthropicEngine to clean up the codebase.

Fixes Issues

• Closes #
• Closes #

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed