Zarr V3 metadata fixes

[LDeakin]

This fixes several problems with zarr.json metadata that I noticed when implementing a chunk manifest storage transformer.

• Chunk key encoding should be v2 with . separator to match manifest.json
  • the chunk-manifest-json storage transformer should not need to be aware of the chunk key encoding
• Fix the fill value defaulting to NaN for integer arrays
• Encode non-finite float fill values as strings
• Default bytes codec "endian" to "little"
  • This needs to be addressed properly at some point
• Fix tests using null fill value or nan fill value for integer data type

[LDeakin]

Type-checking failures seem unrelated