[AudioTranscoding] Add FLAC as supported target audio format and be able to define the corresponding target sample rate

MediaBrowser.MediaEncoding/Encoder/EncoderValidator.cs

@@ -25,6 +25,7 @@ namespace MediaBrowser.MediaEncoding.Encoder
             "ac3",
             "aac",
             "mp3",
+            "flac",
             "h264_qsv",
             "hevc_qsv",
             "mpeg2_qsv",
@@ -71,6 +72,7 @@ namespace MediaBrowser.MediaEncoding.Encoder
             "libmp3lame",
             "libopus",
             "libvorbis",
+            "flac",
             "srt",
             "h264_amf",
             "hevc_amf",