Adds session state management for the credentials returned by the underlying
agent client, such that on agent client initialization, the returned credentials
are saved in session storage. When chat-headless is reloaded and getNextMessage
is called, the session in the agent client is reinitialized.

Session credentials are cleared any time the client is reset.

TEST=manual,auto

Wrote new unit tests, saw them pass.

Ran local test app with a Zendesk client, saw conversation persisted
across page refreshes.

… github action