Add resource configuration to disable automatic ID generation

[wolfgang42]

I have some resources whose IDs are integers auto-generated by the database server. I declare them as:

jsonApi.define({
    resource: 'Order',
    attributes: {
        id: jsonApi.Joi.number().integer(),
        // Other attributes
    },
    handler: customHandler,
})

This causes a problem when I try to create a new ID: a UUID is automatically generated, but then fails validation because it is not an integer.
I therefore propose a new key in the resource definition, generateId, which can be set to false to disable autogenerating IDs. I have implemented this on my branch create-generate-id, and it is working well for my use so far.
In the future this key could also be extended to take a function which returns an ID, so a project could define its own ID generation scheme. (For example using shortid; or perhaps even a content-addressable system where a data property is hashed to generate the ID.)

[st3xupery]

I have my own work around for this as an autoIncrement boolean setting in the resource config. I however check and delete the generated id in my own data handler as such:

if (this.autoIncrement) delete newResource.id;

So technically you can program this behavior without modifying the internals of the server. Obviously, by adjusting to this boolean internally feels a little more elegant, you might as well make a PR, I'm biased to the param key of autoIncrement, maybe autoGenerateId is a good hybrid.

[aledalgrande]

This would be nice as in most cases with SQL databases IDs are integers.

[championswimmer]

would so totally love to see this happening