Improve integration with Java's security manager

Description

Currently we wrap all reflection calls in PrivilegedAction. This way Validators need the following grants in the policy file:

grant codeBase "file:/path/to/hibernate-validator-5.1.1.Final.jar" {
        permission java.lang.reflect.ReflectPermission "suppressAccessChecks";
        permission java.lang.RuntimePermission "accessDeclaredMembers";
        ...
    };

However, this also means that a user might now use ReflectionHelper to execute reflection calls which otherwise would be no allowed. To prevent this we need a Validator specific permission type. Something like this:

class ReflectionHelper {
        public static Field getDeclaredField(Class<?> clazz, String fieldName) {
            SecurityManager securityManager = System.getSecurityManager();

            if ( securityManager != null ) {
                securityManager.checkPermission( HibernateValidatorInternalPermission.INSTANCE );
            }
            ...
        }
    }