RequireRole permission for commands. (#2923)

* Create RequireRoleAttribute.cs

* Apply suggestions from code review

---------

Co-authored-by: Mihail Gribkov <61027276+Misha-133@users.noreply.github.com>

src/Discord.Net.Commands/Attributes/Preconditions/RequireRoleAttribute.cs

@@ -0,0 +1,74 @@
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace Discord.Commands
+{
+    /// <summary>
+    ///     Requires the user invoking the command to have a specified role.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
+    public class RequireRoleAttribute : PreconditionAttribute
+    {
+        /// <summary>
+        ///     Gets the specified Role name of the precondition.
+        /// </summary>
+        public string RoleName { get; }
+
+        /// <summary>
+        ///     Gets the specified Role ID of the precondition.
+        /// </summary>
+        public ulong? RoleId { get; }
+
+        /// <summary>
+        ///     Gets or sets the error message if the precondition
+        ///     fails due to being run outside of a Guild channel.
+        /// </summary>
+        public string NotAGuildErrorMessage { get; set; }
+
+        /// <summary>
+        ///     Requires that the user invoking the command to have a specific Role.
+        /// </summary>
+        /// <param name="roleId">Id of the role that the user must have.</param>
+        public RequireRoleAttribute(ulong roleId)
+        {
+            RoleId = roleId;
+        }
+
+        /// <summary>
+        ///     Requires that the user invoking the command to have a specific Role.
+        /// </summary>
+        /// <param name="roleName">Name of the role that the user must have.</param>
+        public RequireRoleAttribute(string roleName)
+        {
+            RoleName = roleName;
+        }
+
+
+        public override Task<PreconditionResult> CheckPermissionsAsync(ICommandContext context, CommandInfo command, IServiceProvider services)
+        {
+            if (context.User is not IGuildUser guildUser)
+                return Task.FromResult(PreconditionResult.FromError(NotAGuildErrorMessage ?? "Command must be used in a guild channel."));
+
+            if (RoleId.HasValue)
+            {
+                if (guildUser.RoleIds.Contains(RoleId.Value))
+                    return Task.FromResult(PreconditionResult.FromSuccess());
+                else
+                    return Task.FromResult(PreconditionResult.FromError(ErrorMessage ?? $"User requires guild role {context.Guild.GetRole(RoleId.Value).Name}."));
+            }
+
+            if (!string.IsNullOrEmpty(RoleName))
+            {
+                var roleNames = guildUser.RoleIds.Select(x => guildUser.Guild.GetRole(x).Name);
+
+                if (roleNames.Contains(RoleName))
+                    return Task.FromResult(PreconditionResult.FromSuccess());
+                else
+                    return Task.FromResult(PreconditionResult.FromError(ErrorMessage ?? $"User requires guild role {RoleName}."));
+            }
+
+            return Task.FromResult(PreconditionResult.FromSuccess());
+        }
+    }
+}