Feature/extend joi validation

[maldimirov]

Resolves #65 + some small additions

• Able to extend the default Joi validation schema produced by enjoi
• Make console warning on empty schema configurable through options - an empty schema or one that is considered empty is valid, so the warning should be removable. Refer to https://json-schema.org/latest/json-schema-core.html#rfc.section.4.3.1. The default value of the option is left to true to avoid breaking changes.
• Tests and documentation covering the new functionality

[tlivings]

Concerned this sounds a lot like joi extensions.

[maldimirov]

It sound similar, but the key is in the details. Maybe it can be called extendJoiSchemas with a bit addition to the description:

extendJoiSchemas - gives the ability to extend the default Joi validation schema produced by Enjoi. Works for the root schema and for referenced ones. In contrast to extensions which extends on a per type level, this options extends on a per schema level. Useful when you want the extend the Joi schema produced by Enjoi, without defining a new custom type.

[tlivings]

Hard to use this as an example to illustrate why this is useful. Could use currently supported json-schema to specify.

[maldimirov]

You are right. What do you think about this example:

const schema = Enjoi.schema({
    type: 'object',
    required: ['min', 'max'],
    properties: {
        lower: {type: 'number', minimum: 5},
        higher: {type: 'number', maximum: 20},
    },
}, {
    extendValidation: {
        '#': Joi.object().keys({
            higher: Joi.number().greater(Joi.ref('lower')),
        }),
    },
});

[tlivings]

I wonder if the best way to support this is to use the concept of "vendor" extensions.

For example:

{
  "type": "string",
  "x-additional-validation": ... capture somehow
}

[tlivings]

Maybe, just enable something that lets people have a general catch-all for an x- attribute which will invoke a function on an object under that key, pass the current schema to it. Could be used for almost anything - not just extending.

[maldimirov]

Well the original idea (which was inspired by the needs of our project) was to stay as close to native JSON Schema and Joi as possible. x- attributes are not exactly native JSON Schema, so they conflict with this idea. We can also extend the validation by writing validation logic ourselves outside of Enjoi. I don't see what advantages would x- attributes bring us in that case.
The advantage of using Joi is the standardization of the validation definitions and the aggregation of all validation errors in one place. The latter being the bigger advantage and the main motivation for this PR.

[tlivings]

I think at this point this needs to be closed. I believe this feature would now be supported by recent merged refineSchema as well.