refactor(cdn): CDN URL 변환 함수를 공통화한다

src/main/kotlin/kr/co/vividnext/sodalive/v2/common/domain/CdnUrlExtensions.kt

@@ -0,0 +1,7 @@
+package kr.co.vividnext.sodalive.v2.common.domain
+
+fun String?.toCdnUrl(cloudFrontHost: String): String? {
+    if (isNullOrBlank()) return null
+    if (startsWith("https://") || startsWith("http://")) return this
+    return "$cloudFrontHost/$this"
+}