Add endpoint for adding pooled beatmaps to a tournament

[hburn7]

Closes #510 (except it adds to the list instead of overwriting it)

• Adds /tournaments/{id}/beatmaps POST endpoint which accepts an ICollection<long> osuBeatmapIds collection.
• Each osu! id in the collection gets converted into a new beatmap if it doesn't exist.
• Each beatmap (either existing or newly created) is then added to the Tournament's PooledBeatmaps collection. This creates an entry in the __join__pooled_beatmaps table automatically.
• The endpoint returns the current list of pooled beatmaps for the tournament after the modifications.
• This implementation ignores duplicates and does not unnecessarily create join table entries or beatmap table entries.

I am not concerned with the performance of this endpoint despite it requiring multiple round trips per query.

[github-actions[bot]]

Qodana for .NET

1 new problem were found

Inspection name	Severity	Problems
Redundant using directive	🔶 Warning	1

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[myssto]

Since I don't really have a better solution in mind off the top of my head I'm just gonna air this out and see what you think. First thing that comes to mind that bugs me is that we are returning the beatmap DTOs after creation. For any newly created beatmaps, these will be completely useless objects with empty fields for everything, and then it looks like we're also combining these with beatmaps that already exist.

So it feels a bit improper but what do you think? In my head there are a couple solutions I can think of right now:

• Map and return only the newly created beatmaps to a CreatedDTO<T> and return them with a 201 response instead of 200
• Return no response body (bleh)

Either way I think regardless of what we do we should better document what exactly this endpoint is returning since from an API consumer point of view its super super unclear

[hburn7]

I can't think of a use case where the beatmap created result would be used so I would prefer to just return 200.