Create an option to store Anthropic API key in the VS Code secret store

[petecheslock]

Currently in VS code - when you click the gear icon to change your model you have an option to add your own openAI key - This uses the VS code commands to properly store the key in the vscode secret storage.

When the Anthropic support (getappmap/appmap-js#1933) is released in the Navie RPC client we should add a way to securly store the API key for the user so they don't need to save it as an environment variable.