Chat: Add configuration for Ollama chat server

[abeatrix]

Follow up on #3282, #3282 (comment) in specific, to make the URL for the chat completion endpoint to be configurable. Default to use the default Ollama local host.

• Add experimentalOllamaChatApiEndpoint to Configuration interface
• Update ollamaChatClient to use configurable API endpoint
• Add apiEndpoint to CompletionParameters interface
• Add cody.experimental.ollamaChat.ApiEndpoint setting to VSCode extension
• Pass configured API endpoint to chatClient.chat in SimpleChatPanelProvider

This allows using a custom API endpoint for the Ollama chat models, enabling cases where the chat server is hosted remotely, or separately from their autocompletion server

Test plan

Update the new configuration to see if the chat works based on the change

[github-actions]

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