This commit makes the "spring.session.store-type" property optional,
adding an additional check when it is not present that validates only
one supported implementation is available on the classpath.

As Spring Session has been modularized, the chance that multiple
implementations are available on the classpath are lower. When only
one implementation is present, we attempt to auto-configure it. When
more than one implementation is present and no session store is
configured, a NonUniqueSessionRepositoryException is thrown.

Closes gh-9863