feat(bot): allow separate member avatars for proxied messages (#523)

This allows for using one avatar for the member card, and a different
avatar for proxied messages - so that users can set the main avatar to
a "full" version of their avatar, and the "proxy" avatar to a cropped
version.

PluralKit.Core/Database/Functions/ProxyMember.cs

@@ -23,9 +23,9 @@ public class ProxyMember
     public string Name { get; } = "";
 
     public string? ServerAvatar { get; }
+    public string? WebhookAvatar { get; }
     public string? Avatar { get; }
 
-
     public bool AllowAutoproxy { get; }
     public string? Color { get; }
 
@@ -42,5 +42,5 @@ public class ProxyMember
         return memberName;
     }
 
-    public string? ProxyAvatar(MessageContext ctx) => ServerAvatar ?? Avatar ?? ctx.SystemAvatar;
+    public string? ProxyAvatar(MessageContext ctx) => ServerAvatar ?? WebhookAvatar ?? Avatar ?? ctx.SystemAvatar;
 }