Add CBOR value codec

[masih]

Add COBR value codec and benchmarks that compare it with the existing JSON and custom binary format. Set CBOR as default if its performance is comparable with the custom binary format in favour of using more common data formats.

[gammazero]

If there is never a consumer of the valuestore's index data, outside of our own indexer-core implementation, then I suggest that we do not need this, and this issue should be closed.

If someone wants to write their own custom core by implementing the interfaces in this repo, then they are welcome to include whatever codec they want. It is not necessary to offer them a full menu of codecs to choose from here.