feat: extend SurfCoreApi and add Velocity-specific implementation

- Add `getOfflinePlayer(uuid)` method to `SurfCoreApi` and its implementations.
- Update `getCurrentServerName()` to return non-nullable String.
- Introduce `SurfCoreApiVelocityImpl` for Velocity platform with server name retrieval logic.

Author: TheBjoRedCraft

surf-core-api/surf-core-api-common/src/main/kotlin/dev/slne/surf/core/api/common/SurfCoreApi.kt

@@ -12,7 +12,8 @@ interface SurfCoreApi {
     fun getPlayer(name: String): SurfPlayer?
     fun getPlayer(uuid: UUID): SurfPlayer?
 
-    fun getCurrentServerName(): String?
+    fun getCurrentServerName(): String
 
     suspend fun getOfflinePlayer(name: String): SurfPlayer?
+    suspend fun getOfflinePlayer(uuid: UUID): SurfPlayer?
 }

surf-core-core/surf-core-core-common/src/main/kotlin/dev/slne/surf/core/core/common/SurfCoreApiImpl.kt

@@ -15,4 +15,7 @@ abstract class SurfCoreApiImpl : SurfCoreApi {
 
     override suspend fun getOfflinePlayer(name: String) =
         surfPlayerService.getOrLoadPlayerByName(name)
+
+    override suspend fun getOfflinePlayer(uuid: UUID) =
+        surfPlayerService.getOrLoadPlayerByUuid(uuid)
 }

surf-core-velocity/src/main/kotlin/dev/slne/surf/core/velocity/api/SurfCoreApiVelocityImpl.kt

@@ -0,0 +1,12 @@
+package dev.slne.surf.core.velocity.api
+
+import com.google.auto.service.AutoService
+import dev.slne.surf.core.api.common.SurfCoreApi
+import dev.slne.surf.core.core.common.SurfCoreApiImpl
+import dev.slne.surf.core.velocity.surfServerConfig
+import net.kyori.adventure.util.Services
+
+@AutoService(SurfCoreApi::class)
+class SurfCoreApiVelocityImpl : SurfCoreApiImpl(), Services.Fallback {
+    override fun getCurrentServerName() = surfServerConfig.serverName
+}