Implement supplementary directives for all Eloquent relationship types

Is your feature request related to a problem? Please describe.

Many users have come up with use cases where they require some of the other relationship types of Eloquent, such as Polymorphic relationships or Pivot Relations. In many cases, they work fine without any directives or do also work when using @belongsTo or @hasMany, although edge cases do come up from time to time.

To clarify: Other relationship types still work without those directives. They can simply be declared as fields on the type without using any directives.

Describe the solution you'd like

Implement additional directives for Eloquent's other relationship types. I am sure we can
do some nice optimizations and make Lighthouse even more simple to use.

https://laravel.com/docs/eloquent-relationships#defining-relationships

Support

• hasMany
• belongsTo
• hasOne
• belongsToMany
• hasOneThrough
• hasManyTrough
• morphTo
• morphOne
• morphMany
• morphToMany
• morphedByMany

No source files or tests are named in the issue. Start by comparing the existing Eloquent relationship directives with Laravel's relationship documentation, focusing on the unchecked morphedByMany type; done means the additional directive is implemented and its edge cases are covered consistently with the supported relationship directives.