Works quite similarly to vscode.dev. The client has a key stored in
secret storage. The server has a key stored server-side, and issues
an http-only cookie to the client. The client can ask the server to
combine its key and the http-only cookie key to a key component, which
it combines with its local key to encrypt and decrypt data.
This logic kicks in if the web server bits see a `vscode-secret-key-path`
cookie set when it loads.