Add support for used in endpoints

[winklertomas]

Motivation

Add support for new used-in delivery API feature.

Checklist

• Code follows coding conventions held in this repo
• Automated tests have been added
• Tests are passing
• Docs have been updated (if applicable)
• Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

Release date

Please do not release before the used-in is released in the production. The early access should be released in the middle of March, we will let you know.

[pokornyd]

add null handling and consider changing EnumerateFeed to async

[pokornyd]

existing methods accepting a codename parameter have null handling in place. consider adding it to all usedIn methods as well:

if (codename == null)
{
    throw new ArgumentNullException(nameof(codename), "The codename of a content item is not specified.");
}

[pokornyd]

also null handling:

if (codename == null)
{
    throw new ArgumentNullException(nameof(codename), "The codename of an asset is not specified.");
}

[pokornyd]

this method is synchronous but invokes an async method and waits for its result, effectively blocking threads. not sure if this is a valid scenario under the circumstances, but I'm not familiar with Rx so I asked AI about it.

the fix is simple, make the method async and have it return IAsyncEnumerable:

private static async IAsyncEnumerable<T> EnumerateFeed<T>(IDeliveryItemsFeed<T> feed) where T : class
{
    while (feed.HasMoreResults)
    {
        var batch = await feed.FetchNextBatchAsync();
        foreach (var contentItem in batch.Items)
        {
            yield return contentItem;
        }
    }
}

however, related methods would need to be updated as well.

again, I'm not sure if thread locking or even deadlocks can be a thing in our scenario. also, Rx is used much less, compared to async, so I leave this at your decision.