adds extensions that zarr-python defines

[normanrz]

Codecs:

• vlen-utf8
• vlen-bytes

Data types:

• string
• bytes

[jhamman]

@normanrz - what's the path to merge this look like? I ask because I'd like to move #5 forward and the fact that this has sat for two months has me a bit worried.

[normanrz]

Since these are the first extensions to be registered, I wanted to stall these first PRs until zarr-developers/zarr-specs#330 is merged to give people some time to give feedback on the extension process.
Your PR looks good and I don't see why it couldn't be merged once 330 is in.