added botonguild api endpoint

src/EllieBot/Services/GrpcApi/OtherSvc.cs

@@ -39,6 +39,19 @@ public sealed class OtherSvc : GrpcOther.GrpcOtherBase, IEService
         _cache = cache;
     }
 
+    [GrpcNoAuthRequired]
+    public override async Task<BotOnGuildReply> BotOnGuild(BotOnGuildRequest request, ServerCallContext context)
+    {
+        var guild = await _client.GetGuildAsync(request.GuildId);
+
+        var reply = new BotOnGuildReply
+        {
+            Success = guild is not null
+        };
+
+        return reply;
+    }
+
     public override async Task<GetTextChannelsReply> GetTextChannels(
         GetTextChannelsRequest request,
         ServerCallContext context)