Add User-Agent to request

We generate a User-Agent for the platform we are running on and add it to the URLTester requests and also to MPV and AVPlayer

Model/Player/Backends/AVPlayerBackend.swift

@@ -232,7 +232,10 @@ final class AVPlayerBackend: PlayerBackend {
         upgrading: Bool = false
     ) {
         asset?.cancelLoading()
-        asset = AVURLAsset(url: url)
+        asset = AVURLAsset(
+            url: url,
+            options: ["AVURLAssetHTTPHeaderFieldsKey": ["User-Agent": "\(UserAgentManager.shared.userAgent)"]]
+        )
         asset?.loadValuesAsynchronously(forKeys: Self.assetKeysToLoad) { [weak self] in
             var error: NSError?
             switch self?.asset?.statusOfValue(forKey: "duration", error: &error) {

Model/Player/Backends/MPVClient.swift

@@ -68,6 +68,7 @@ final class MPVClient: ObservableObject {
         checkError(mpv_set_option_string(mpv, "vo", "libmpv"))
         checkError(mpv_set_option_string(mpv, "demuxer-lavf-analyzeduration", "1"))
         checkError(mpv_set_option_string(mpv, "deinterlace", Defaults[.mpvDeinterlace] ? "yes" : "no"))
+        checkError(mpv_set_option_string(mpv, "user-agent", UserAgentManager.shared.userAgent))
 
         checkError(mpv_initialize(mpv))