Support validation against cached namespace via flag on pynwb.validate

[oruebel]

The pynwb.validate function currently validates by default against the core namespace, whereas the command line validator by default uses the cached namespace. The functionality to use cached namespaces should be moved/ported to the validate function.

This is related to the following DANDI issue https://github.com/dandi/dandi-cli/issues/917

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