Add dictionary-in-stream format in contrib

[yotann]

Adds a format in contrib/dict_in_stream that stores a dictionary, optionally compressed, in a skippable frame at the beginning of a stream. This format is useful to reduce the size of streams that have many frames. For instance, Wget-AT is using this format to compress WARC files using one frame per record and allow random access.

cc @JustAnotherArchivist