BeanProvider: add convenience methods without the 'optional' flag

The BeanProvider methods
#getContextualReference(String name, boolean optional) and
#getContextualReference(Type t, boolean optional, Annotation... qualifiers)
will in 90% of the cases get called with optional=false to throw an IllegalStateException if no bean could be found.

Thus we shall introduce 2 convenient methods without the boolean parameter which will fail if none of the requested beans exist.

We don't need to do this for methods like getContextualReferences (plural) as those methods are not used that frequently.