Update blob file format for ZSTD dictionary compression (#109)

Adding a new version to blob file format. v2 blob files come with 4 more bytes in the header to hold future flags. In particular if `kHasUncompressionDictionary` is set, the whole file is compressed using dictionary compression and the per-blob compression type should be ignored.

Also update `BlobEncoder` and `BlobDecoder` to allow passing in a dictionary and use it to  compress/decompress. Add unit test for it.