Implement Stage instances (#291)

* Add low-level implementation of stage instances

* Add helper functions

* Add core entities and api representations

* Expose creation of StageInstanceBehavior to unsafe
- Revert outdated change

* Final additions
- Add StageInstanceBehavior.asStageInstance
- Fix compiler issue
- Add StageChannelBehavior.getStageInstance()

* Add StageInstances to EntitySupplier.kt

* Add StageInstances to EntitySupplier.kt

* Fix typo

* Apply requested changes

core/src/main/kotlin/Unsafe.kt

@@ -68,6 +68,10 @@ class Unsafe(private val kord: Kord) {
     fun webhook(id: Snowflake): WebhookBehavior =
         WebhookBehavior(id, kord)
 
+    fun stageInstance(id: Snowflake, channelId: Snowflake): StageInstanceBehavior = StageInstanceBehavior(
+        id, channelId, kord, kord.defaultSupplier
+    )
+
     override fun toString(): String {
         return "Unsafe"
     }
@@ -87,5 +91,4 @@ class Unsafe(private val kord: Kord) {
     ): GlobalApplicationCommandBehavior =
         GlobalApplicationCommandBehavior(applicationId, commandId, service)
 
-
 }

core/src/main/kotlin/behavior/channel/StageChannelBehavior.kt

@@ -3,7 +3,6 @@ package dev.kord.core.behavior.channel
 import dev.kord.common.entity.Snowflake
 import dev.kord.core.Kord
 import dev.kord.core.cache.data.ChannelData
-import dev.kord.core.entity.channel.VoiceChannel
 import dev.kord.core.cache.data.StageInstanceData
 import dev.kord.core.entity.StageInstance
 import dev.kord.core.entity.channel.Channel
@@ -33,6 +32,7 @@ interface StageChannelBehavior : BaseVoiceChannelBehavior {
         return StageChannelBehavior(id, guildId, kord, strategy.supply(kord))
     }
 
+
     suspend fun createStageInstance(topic: String): StageInstance {
         val instance = kord.rest.stageInstance.createStageInstance(id, topic)
         val data = StageInstanceData.from(instance)

core/src/samples/kotlin/PingBot.kt

@@ -1,4 +1,8 @@
 import dev.kord.core.Kord
+import dev.kord.core.behavior.channel.createStageInstance
+import dev.kord.core.behavior.channel.getStageInstance
+import dev.kord.core.behavior.getChannelOf
+import dev.kord.core.entity.channel.StageChannel
 import dev.kord.core.event.message.MessageCreateEvent
 import dev.kord.core.on