Relationship URI should be moved to schema relation field

[lindyhopchris]

At the moment, if you want to use a different convention for a relationship's URI, you have to define it in two places.

Firstly, when registering the route:

$relationships->hasOne('blogPost')->uri('blog_post');

Secondly, on the resource's relation object:

$this->relation('blogAuthor')->retainFieldName()
// or
$this->relation('blogAuthor')->withUriFieldName('blog_author')

Now that we have made resource classes optional, we should move this setting to the schema's relation field. That would mean we could use it both when serializing the relation and when registering routes. (To use it when serializing relations we would need to always inject the schema into the resource class - even if using a specific resource class e.g. PostResource.)