Improve avatar lookup for IRC->Discord messages via webhook.

We will now lookup avatars for IRC message senders by first looking for
a matching Discord username, if we don't find one with a matching
username, we will then check the user's nickname.

I expect this is usually preferred, but if not we can investigate an
option or alternate behavior.

We will also now fire off a task to query Discord for matching users
rather than only relying on the bot's local user cache. This means that
if someone speaks in IRC who has not yet spoken in Discord, we should
automatically pull down their avatar by the second or third message
(depending on network conditions, API, guild size, other factors).

This should mean that users no longer need to speak in Discord after
the bot is restarted; we should figure out their name within a few
messages. It is also much cheaper from a memory standpoint than always
aggressively caching the entire guild (especially on very large guilds).

src/main/kotlin/io/zachbr/dis4irc/bridge/pier/discord/DiscordPier.kt

@@ -24,6 +24,7 @@ import net.dv8tion.jda.api.JDABuilder
 import net.dv8tion.jda.api.entities.Activity
 import net.dv8tion.jda.api.entities.Activity.ActivityType
 import net.dv8tion.jda.api.entities.Guild
+import net.dv8tion.jda.api.entities.Member
 import net.dv8tion.jda.api.entities.channel.concrete.TextChannel
 import net.dv8tion.jda.api.requests.GatewayIntent
 import net.dv8tion.jda.api.utils.MemberCachePolicy
@@ -157,12 +158,8 @@ class DiscordPier(private val bridge: Bridge) : Pier {
     }
 
     private fun sendMessageWebhook(guild: Guild, webhook: WebhookClient, msg: Message) {
-        // try and get avatar for matching user
-        var avatarUrl: String? = null
-        val matchingUsers = guild.getMembersByEffectiveName(msg.sender.displayName, true)
-        if (matchingUsers.isNotEmpty()) {
-            avatarUrl = matchingUsers.first().user.avatarUrl
-        }
+        val guildUser = getMemberByUserNameOrDisplayName(msg.sender.displayName, guild)
+        var avatarUrl = guildUser?.effectiveAvatarUrl
 
         var senderName = enforceSenderName(msg.sender.displayName)
         // if sender is command, use bot's actual name and avatar if possible
@@ -258,6 +255,24 @@ class DiscordPier(private val bridge: Bridge) : Pier {
         }
     }
 
+    private fun getMemberByUserNameOrDisplayName(name: String, guild: Guild, ignoreCase: Boolean = true): Member? {
+        // check by username first
+        var matchingUsers = guild.getMembersByName(name, ignoreCase)
+        // if no results, check by their nickname instead
+        if (matchingUsers.isEmpty()) {
+            matchingUsers = guild.getMembersByNickname(name, ignoreCase)
+        }
+        // if we still don't have any results, fire off a findMembers call to look it up (and cache it for later)
+        // this won't help us with this specific call (we don't really want to wait around for this task to come back),
+        // but it will help us with future calls to this and other functions, so the next time they talk, we'll have it.
+        if (matchingUsers.isEmpty()) {
+            guild.findMembers { it.user.name.equals(name, ignoreCase) || it.nickname.equals(name, ignoreCase) }
+                .onSuccess { logger.debug("Cached ${it.size} results for user lookup: $name") }
+        }
+
+        return matchingUsers.firstOrNull()
+    }
+
     /**
      * Gets an enum from the given string or throw an IllegalArgumentException with the given error message
      */