PHP 8 support and Schema annotation improvements

Allow referencing previously defined named types (#51)

Once a named type is defined, in order to reuse that same type it is
needed to reference that type by the name (as in the primitive types).

Add support for NamedTypes in both the schema builder and the schema
generator, which is a way of referencing previously defined named types.

Add support for adding extra attributes for records in field's
definition.