cbs_av1: Don't reject unknown metadata

Accept it and pass it through unchanged.

The standard requires that decoders ignore unknown metadata, and indeed
this is tested by some of the Argon coverage streams.

libavcodec/cbs_av1.h

@@ -370,6 +370,12 @@ typedef struct AV1RawMetadataTimecode {
     uint32_t time_offset_value;
 } AV1RawMetadataTimecode;
 
+typedef struct AV1RawMetadataUnknown {
+    uint8_t     *payload;
+    AVBufferRef *payload_ref;
+    size_t       payload_size;
+} AV1RawMetadataUnknown;
+
 typedef struct AV1RawMetadata {
     uint64_t metadata_type;
     union {
@@ -378,6 +384,7 @@ typedef struct AV1RawMetadata {
         AV1RawMetadataScalability scalability;
         AV1RawMetadataITUTT35     itut_t35;
         AV1RawMetadataTimecode    timecode;
+        AV1RawMetadataUnknown     unknown;
     } metadata;
 } AV1RawMetadata;