[Feature] Premium subscriptions (#2781)

* what a big commit lel, add app sub enums

* work

* ah yup lol

* `?`

* events 1

* typo

* `list` => `get` | remaining events

* add `RespondWithPremiumRequiredAsync` to interaction module base

src/Discord.Net.WebSocket/Entities/AppSubscriptions/SocketEntitlement.cs

@@ -0,0 +1,91 @@
+using Discord.Rest;
+using System;
+
+using Model = Discord.API.Entitlement;
+
+namespace Discord.WebSocket;
+
+public class SocketEntitlement : SocketEntity<ulong>, IEntitlement
+{
+    /// <inheritdoc/>
+    public DateTimeOffset CreatedAt { get; private set; }
+
+    /// <inheritdoc/>
+    public ulong SkuId { get; private set; }
+
+    /// <inheritdoc cref="IEntitlement.UserId"/>
+    public Cacheable<SocketUser, RestUser, IUser, ulong>? User { get; private set; }
+
+    /// <inheritdoc cref="IEntitlement.GuildId"/>
+    public Cacheable<SocketGuild, RestGuild, IGuild, ulong>? Guild { get; private set; }
+
+    /// <inheritdoc/>
+    public ulong ApplicationId { get; private set; }
+
+    /// <inheritdoc/>
+    public EntitlementType Type { get; private set; }
+
+    /// <inheritdoc/>
+    public bool IsConsumed { get; private set; }
+
+    /// <inheritdoc/>
+    public DateTimeOffset? StartsAt { get; private set; }
+
+    /// <inheritdoc/>
+    public DateTimeOffset? EndsAt { get; private set; }
+
+    internal SocketEntitlement(DiscordSocketClient discord, ulong id) : base(discord, id)
+    {
+    }
+
+    internal static SocketEntitlement Create(DiscordSocketClient discord, Model model)
+    {
+        var entity = new SocketEntitlement(discord, model.Id);
+        entity.Update(model);
+        return entity;
+    }
+
+    internal void Update(Model model)
+    {
+        SkuId = model.SkuId;
+
+        if (model.UserId.IsSpecified)
+        {
+            var user = Discord.GetUser(model.UserId.Value);
+
+            User = new Cacheable<SocketUser, RestUser, IUser, ulong>(user, model.UserId.Value, user is not null, async ()
+                    => await Discord.Rest.GetUserAsync(model.UserId.Value));
+        }
+
+        if (model.GuildId.IsSpecified)
+        {
+            var guild = Discord.GetGuild(model.GuildId.Value);
+
+            Guild = new Cacheable<SocketGuild, RestGuild, IGuild, ulong>(guild, model.GuildId.Value, guild is not null, async ()
+                => await Discord.Rest.GetGuildAsync(model.GuildId.Value));
+        }
+
+        ApplicationId = model.ApplicationId;
+        Type = model.Type;
+        IsConsumed = model.IsConsumed;
+        StartsAt = model.StartsAt.IsSpecified
+            ? model.StartsAt.Value
+            : null;
+        EndsAt = model.EndsAt.IsSpecified
+            ? model.EndsAt.Value
+            : null;
+    }
+
+    internal SocketEntitlement Clone() => MemberwiseClone() as SocketEntitlement;
+
+    #region IEntitlement
+
+    /// <inheritdoc/>
+    ulong? IEntitlement.GuildId => Guild?.Id;
+
+    /// <inheritdoc/>
+    ulong? IEntitlement.UserId => User?.Id;
+
+    #endregion
+
+}