Make possible to add NFTs to existing featured NFTs

[drillprop]

Context
Currently, an operator can only set an array of ids, and once he set them, all the previously featured NFTs will be overwritten. For example, if an operator runs:

mutation {
    setFeaturedNfts(featuredNftsIds: ["1", "2"]) {
        newNumberOfNftsFeatured
    }
}

and then let's say he wants to add more NFTs to the existing set, If he runs:

mutation {
    setFeaturedNfts(featuredNftsIds: ["3", "4"]) {
        newNumberOfNftsFeatured
    }
}

The featured NFTs will be only “3” and “4”.

To fix this he needs to provide all the ids, so in the case above, he would have to write this mutation:

mutation {
    setFeaturedNfts(featuredNftsIds: ["1", "2", "3", "4"]) {
        newNumberOfNftsFeatured
    }
}

Solution
Introduce:

• addFeaturedNfts mutation
• removeFeaturedNfts mutation

[dmtrjsg]

@ignazio-bovo assigned to you to get this merged (was reviewed already but at this point there likely be conflicts that require resolution).

This is lower prio compared to Notifications and change requests for CRTs.

[dmtrjsg]

we don't need to do it, but since its done then better to merge :)