Ability to specify default includes if the client did not send any include

[X-Coder264]

In the previous package which used Neomerx v1 encoder we had getIncludePaths method in the schema (https://github.com/neomerx/json-api/blob/v1.0.9/src/Schema/SchemaProvider.php#L236) to specify which includes would get sent back to the client if the client did not explicitly provide any includes. It'd be nice to have the same feature in this package.

[lindyhopchris]

My comment from Slack describing how this should be implemented:

Yeah this one is very simple. If you add a $includePaths property to the Query and QueryCollection classes, then in the includePaths() method, if the value from the client is null you return the value from the include paths property instead. The $includePaths property can be of mixed type: you cast it to the value object using IncludePaths::nullable($this->includePaths) - hopefully that'll make sense when you look at the class.