support plain string form for codecs, data types, etc

[d-v-b]

Version 3.1 of the Zarr V3 spec introduced the option to use plain strings in zarr.json for any {"name": <>, "configuration": <>} data structure if the "configuration" field is optional. It's also possible for such JSON objects to contain an new key, "must_understand", which is a boolean. See discussion of these changes here.

We already have an open PR that aims to address this: #2866

[d-v-b]

specifically, for e.g. the bytes codec, we need to support the following variants when reading metadata:

    "bytes"
    {"name": "bytes"} 
    {"name": "bytes", "configuration": {}}
    {"name": "bytes", "configuration": {"endian": "little"}}
    {"name": "bytes", "configuration": {"endian": "big"}}

and for each of the JSON object forms, we need to ensure that the "must_understand" key, if present, is true.