feature: Add ability to modify the banner for guilds (#1432)

src/Discord.Net.Core/Entities/Guilds/GuildProperties.cs

@@ -38,6 +38,10 @@ namespace Discord
         /// </summary>
         public Optional<Image?> Icon { get; set; }
         /// <summary>
+        ///     Gets or sets the banner of the guild.
+        /// </summary>
+        public Optional<Image?> Banner { get; set; }
+        /// <summary>
         ///     Gets or sets the guild's splash image.
         /// </summary>
         /// <remarks>

src/Discord.Net.Rest/API/Rest/ModifyGuildParams.cs

@@ -22,6 +22,8 @@ namespace Discord.API.Rest
         public Optional<ulong?> SystemChannelId { get; set; }
         [JsonProperty("icon")]
         public Optional<Image?> Icon { get; set; }
+        [JsonProperty("banner")]
+        public Optional<Image?> Banner { get; set; }
         [JsonProperty("splash")]
         public Optional<Image?> Splash { get; set; }
         [JsonProperty("afk_channel_id")]

src/Discord.Net.Rest/Entities/Guilds/GuildHelper.cs

@@ -32,6 +32,7 @@ namespace Discord.Rest
                 Icon = args.Icon.IsSpecified ? args.Icon.Value?.ToModel() : Optional.Create<ImageModel?>(),
                 Name = args.Name,
                 Splash = args.Splash.IsSpecified ? args.Splash.Value?.ToModel() : Optional.Create<ImageModel?>(),
+                Banner = args.Banner.IsSpecified ? args.Banner.Value?.ToModel() : Optional.Create<ImageModel?>(),
                 VerificationLevel = args.VerificationLevel,
                 ExplicitContentFilter = args.ExplicitContentFilter,
                 SystemChannelFlags = args.SystemChannelFlags
@@ -57,6 +58,8 @@ namespace Discord.Rest
             else if (args.RegionId.IsSpecified)
                 apiArgs.RegionId = args.RegionId.Value;
 
+            if (!apiArgs.Banner.IsSpecified && guild.BannerId != null)
+                apiArgs.Banner = new ImageModel(guild.BannerId);
             if (!apiArgs.Splash.IsSpecified && guild.SplashId != null)
                 apiArgs.Splash = new ImageModel(guild.SplashId);
             if (!apiArgs.Icon.IsSpecified && guild.IconId != null)