Making Codec classes self-contained

[normanrz]

• Remove the CodecMetadata classes. All codecs contain their confirguration directly and have from_dict and to_dict methods.
• Moves ChunkKeyEncoding and ChunkGrid classes in new modules.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[d-v-b]

In the interest of simplicity, I have a preference for just using a (validated) list of codecs here instead of a special-purpose CodecPipeline object. As I understand it, a CodecPipeline is essentially a (validated) list of codecs, plus some methods on a that list, and those methods could exist as plain functions that take List[Codec] as their first argument. What would be the downsides of that approach?

[d-v-b]

I don't think we need to answer this question here, so I'm going to move ahead with this PR

[normanrz]

I guess the downside is redundancy. We need the CodecPipeline anyways and likely want to cache it. So we can just create it upon construction and use it for validation etc.