Added ability to control field visibility on schema definition

[carlagouveia]

This PR aims to add the ability to control field visibility on the schema definition, using either a bool or a callable that has access to the context. This is not final in any way and I am open to other ideas on how to handle this.

Fixes #782

[carlagouveia]

@spawnia just added the field visibility validation. Let me know if that is what you had in mind. 👍

[spawnia]

I made some modifications to further align how a non-visible and a non-existent field would behave.

Due to the different call sites, we don't always have the same $context available. I think it would be of very limited use to provide whatever context is available, as the presented information can differ and the implementation of visibility would have to account for that. If you think something in particular would be useful or necessary to have available, we can discuss that further.

Please document this feature at https://github.com/webonyx/graphql-php/blob/master/docs/type-definitions/object-types.md#field-configuration-options. The note for visible should provide a short outline why this would be preferrable over conditional inclusion or exclusion of a field, and how exactly it behaves.

[carlagouveia]

I made some modifications to further align how a non-visible and a non-existent field would behave.

Due to the different call sites, we don't always have the same $context available. I think it would be of very limited use to provide whatever context is available, as the presented information can differ and the implementation of visibility would have to account for that. If you think something in particular would be useful or necessary to have available, we can discuss that further.

Please document this feature at https://github.com/webonyx/graphql-php/blob/master/docs/type-definitions/object-types.md#field-configuration-options. The note for visible should provide a short outline why this would be preferrable over conditional inclusion or exclusion of a field, and how exactly it behaves.

@spawnia thanks for all the modifications. I've updated the documentation, adding information about this feature. Here is the PR #1456.

[spawnia]

You can push the docs update directly to this pull request.

[carlagouveia]

You can push the docs update directly to this pull request.

Done @spawnia.