allow extra keywords in structured datatype validation

[braingram]

Description

The ndarray schema doesn't prevent additional keywords in datatype definitions for structured datatypes.
https://github.com/asdf-format/asdf-standard/blob/c17c3ecabcad7b65e41e0fb79054988b37f45a38/resources/schemas/stsci.edu/asdf/core/ndarray-1.1.0.yaml#L214

For example, adding a description to a sub-datatype doesn't violate the schema:

       datatype:
         - name: a
           description: a description
           datatype: int8

However it fails the custom datatype validation in asdf.

This PR updates the custom datatype validator to only compare a selection of datatype keys {"name", "datatype", "byteorder", "shape"}. This allows adding descriptions to sub-datatypes.

Fixes #1900

Tasks

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news fragment change types...

• changes/<PR#>.feature.rst: new feature
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• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.general.rst: infrastructure or miscellaneous change