Implement plugin system (#1020)

* Implement basic plugin system

* The dawn of new era

* Add plugins warning

* Move more members to PublicAPI

* Open commands for the plugins

* Add IBotHackNewChat

* Run plugin events in parallel

* Use properties in IPlugin

* Hook our custom plugin into CI to ensure it compiles

* Fix dotnet brain damage

* Add IBotsComparer

* Add code documentation

* Add IBotTradeOffer

* Add IBotTradeOffer example

* Add IBotTradeOfferResults

* Final bulletproofing

* Final renaming

ArchiSteamFarm/BotConfig.cs

@@ -20,6 +20,7 @@
 // limitations under the License.
 
 using System;
+using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics.CodeAnalysis;
 using System.IO;
@@ -28,7 +29,9 @@ using System.Threading;
 using System.Threading.Tasks;
 using ArchiSteamFarm.Json;
 using ArchiSteamFarm.Localization;
+using JetBrains.Annotations;
 using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
 using SteamKit2;
 
 namespace ArchiSteamFarm {
@@ -142,6 +145,14 @@ namespace ArchiSteamFarm {
 		[JsonProperty(Required = Required.DisallowNull)]
 		internal readonly bool UseLoginKeys = DefaultUseLoginKeys;
 
+		[JsonExtensionData]
+		internal Dictionary<string, JToken> AdditionalProperties {
+			get;
+
+			[UsedImplicitly]
+			private set;
+		}
+
 		internal string DecryptedSteamPassword {
 			get {
 				if (string.IsNullOrEmpty(SteamPassword)) {
@@ -360,9 +371,7 @@ namespace ArchiSteamFarm {
 
 		[Flags]
 		internal enum EBotBehaviour : byte {
-			[SuppressMessage("ReSharper", "UnusedMember.Global")]
 			None = 0,
-
 			RejectInvalidFriendInvites = 1,
 			RejectInvalidTrades = 2,
 			RejectInvalidGroupInvites = 4,
@@ -399,9 +408,7 @@ namespace ArchiSteamFarm {
 
 		[Flags]
 		internal enum ERedeemingPreferences : byte {
-			[SuppressMessage("ReSharper", "UnusedMember.Global")]
 			None = 0,
-
 			Forwarding = 1,
 			Distributing = 2,
 			KeepMissingGames = 4,
@@ -410,9 +417,7 @@ namespace ArchiSteamFarm {
 
 		[Flags]
 		internal enum ETradingPreferences : byte {
-			[SuppressMessage("ReSharper", "UnusedMember.Global")]
 			None = 0,
-
 			AcceptDonations = 1,
 			SteamTradeMatcher = 2,
 			MatchEverything = 4,