PCP-23-1: Update blob file format to accommodate zstd dictionary

[yiwu-arbug]

This is subtask of tikv/tikv#5743.

We need to add a new version to blob file format. If version = 2, it means the blob file come with a zstd dictionary and the dictionary is stored in the meta section (pointed by meta index handle in the footer). Update BlobFileHeader and BlobFileFooter and their comments accordingly.

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

This should be a good warmup task for you to get familiar with Titan development.

[yiwu-arbug]

@foeb