Add ability to filter fk results

[jamienicholls]

You can currently add a foreign key (fk) to a schema, it would be nice to filter that fk on a field that is present as another key in the fk's schema.

i.e. Schema two can only access schema ones data on condition of filter
`
const schemaOne = Joi.object({
schemaOneId: joi.string().pk().required(),
schemaOneCategory: Joi.string(),
isActive: Joi.boolen().meta({}),
};

const schemaTwo = Joi.object({
schemaTwoId: joi.string().pk().required(),
schemaOneItem: Joi.string().fk('schemaOne.[].schemaOneId').meta({{ isActive: { isActive: true } }}),
}

const schema = Joi.object({
schemaOne: Joi.array().items(schemaOne).optional().uniqueOnPks(),
schemaTwo: Joi.array().items(schemaTwo).optional().uniqueOnPks(),
}
`