Fix default subject_types configuration path
    
    Default configuration path for subject_types in message consumers is
    invalid because subject_types directory is not installed in /usr/lib*.
    
    The fix is to use environment variable GREENWAVE_SUBJECT_TYPES_DIR which
    defaults to "/src/conf/subject_types" in the container.
    
    Signed-off-by: Lukas Holecek <hluk@email.cz>