Add @whereHasConditions directive to filter relation existence

[chescos]

• Added or updated tests
• Added Docs for all relevant versions
• Updated CHANGELOG.md

Resolves #1112

Changes

This PR adds a new directive called @whereHasConditions. This directive is very similar to the @whereConstraints directive, except that the query conditions are applied to an Eloquent relationship.

The @whereConstraints directive has also been renamed to @whereConditions after a discussion in #1112.

Here's how the @whereHasConditions directive can be used in the schema:

type Query {
    users(
        hasPosts: _ @whereHasConditions(columns: ["title", "content"])
        hasComments: _ @whereHasConditions(relation: "posts.comments", columns: ["reply"])
    ): [User!]! @all
}

And here's an example query:

{
    users(
        hasPosts: { column: TITLE, operator: LIKE, value: "%some title%" },
        hasComments: { column: REPLY, operator: EQ, value: "hi" }
    ) {
        id
    }
}

Breaking changes

The @whereConstraints directive has been renamed to @whereConditions.

[chescos]

@spawnia I'll start working on this PR soon if that's fine with you. Please let me know if there are any other requirements except what we discussed in #1112.

About renaming the @whereConstraints directive - do you want to do that in a separate PR or should we do this here as well?

[spawnia]

@chescos looking forward to seeing the results, nice! The tests look good.

Let's do the renaming now, i think that will be the least effort overall. It's bettter to make such breaking changes early, before this becomes so great that everybody will use it. We should be able to include it as a stable feature soon.

[chescos]

@spawnia The @whereHasConditions directive is pretty much done, I think I covered everything. The code is mostly copied from the @whereConstraints directive as it follows the same logic. One question though... the Operator and SQLOperator classes can be fully shared between both directives - should they just stay in the WhereConstraints folder?

A code review would be awesome. After that, I'd add the documentation and add it to the changelog.

[spawnia]

All related code should be in a namespace WhereConditions, including everything for the previous @whereConstraints. Having one service provider for both is reasonable.

[chescos]

@spawnia great call, thanks! That made things a lot cleaner already. Anything else that could be improved?

[chescos]

@spawnia could you give it another review?

[spawnia]

This is starting to come together quite nicely. The refactorings and unification has brought the added line count down from ~800 to ~350, which i am much more comfortable with from a maintenance standpoint.

[chescos]

Yes, looking nice so far. Anything else that needs improvement?

[spawnia]

This is good to go, awesome work.

We might consider marking this feature as stable in v5 and including it by default, i really like where it's at now.