Data type extensions

[LDeakin]

In zarrs 0.6.0, data type extensions were disabled because I wasn't sure how to handle them properly in codecs and fallback data types had been removed from the spec. The specification does not consider what should happen with a custom data type in a codec like Bytes, so it must be implementation/user-defined (until it is standardised).

• the Bytes codec needs special handling for custom data types, this can be done with a trait
• the transpose codec trivially supports custom data types (just needs their element size)
• if a custom data type has a fallback numerical data type, codecs like bitround and zfp could still work
• all bytes-to-bytes codecs support custom data types

TODO

• Split out DataType/FillValue into zarrs_data_type crate refactor!: move the zarrs::array::{data_type,fill_value} modules into the zarrs_data_type crate #139
• Add Custom to DataType enum
• Add trait for endianness handling
• Custom data type registration