Adjust pac4j integration to no longer require setting publicAddress launch config property

@rhart pointed out that some of the pac4j changes in 0.9.5 broke the integration when the `publicAddress` launch config property wasn't set.  This was because the `Clients` object was being created as part of the injection configuration, rather than as part of the handler chain.

This change adjusts the pac4j integration such that the `Clients` object is now pulled from the `Context`, as placed there by a `Pac4jClientsHandler`.  If desired, it should be possible to have multiple `Pac4jClientsHandlers` in an application serving different paths.

CC: @leleuj @alkemist