chat-headless: add support for session reinitialization (#59)

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.

packages/chat-headless-react/CONTRIBUTING.md

@@ -1,32 +1,38 @@
 ## Testing Process
 
 ### Unit Testing
+
 We use Jest as our framework for unit tests. Execute the unit tests with the following command:
+
 ```
 npm run test
 ```
 
 ### Test Site
+
 To facilitate manual verification, a React test site has been set up in `apps/test-site`. This site currently already have an App component that interfaces with the local `chat-headless-react` library.
 
 To set up the test site, make sure you have a `.env` file configured following the `.sample.env` file. Then, run the following commands:
+
 ```
 npm i
 npm run start
 ```
 
-
 ## Build Process
 
 Before initiating the build, run the linting process to identify and address any errors or warnings. Use the following command:
+
 ```
 npm run lint
 ```
 
 To build the library, execute:
+
 ```
 npm run build
 ```
+
 This will create the bundle in the `/dist` directory. This command will also generate documentation files and the `THIRD-PARTY-NOTICES` file.
 
-For guidelines on pull request and version publish process, visit Chat SDK wiki page.
+For guidelines on pull request and version publish process, visit Chat SDK wiki page.