avformat/argo_asf: split functionality into a header

avformat/argo_asf: split functionality into a header

For future use by the argo_brp demuxer.

Adds:
  - void ff_argo_asf_parse_file_header(ArgoASFFileHeader *hdr, const uint8_t *buf);
  - int  ff_argo_asf_validate_file_header(AVFormatContext *s, const ArgoASFFileHeader *hdr);
  - void ff_argo_asf_parse_chunk_header(ArgoASFChunkHeader *hdr, const uint8_t *buf);
  - int  ff_argo_asf_fill_stream(AVStream *st, const ArgoASFChunkHeader *ckhdr);

Signed-off-by: Zane van Iperen <zane@zanevaniperen.com>