EZP-23778: implement session-based auth support in BDD RestContext

JIRA: https://jira.ez.no/browse/EZP-23778

This adds support for session-based authentication in BDD's RestContext.
A new `authType` parameter may be added to behat.yml in REST suites to specify which method to use,
current default is "session" as per https://github.com/ezsystems/ezpublish-community/commit/8b1f227e1d84bc78