Add config keys for session base name and cookie expires

[joshcanhelp]

This PR adds the ability to change the base name for the session key as well as the session cookie expires:

• Add a session_base_name key to the config array passed when instantiating Auth0 to change the session key base name used.
• Add a session_cookie_expires key to the config array passed when instantiating Auth0 to change the session cookie expires time. Cannot remove that from the default or else session cookies would be shorter than expected for anyone using the default.
• Moved the SessionStore::initSession() call out of the SessionStore constructor so it happens later in the execution, allowing for apps that start their own sessions to do that more easily.

Closes #215
Closes #273

[cocojoe]

LGTM

[github-actions]

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