Enhance validator to warn on extra fields

[oruebel]

The validator should issue warnings for extra fields, i.e., datasets, attributes, groups etc. that are included in a file that are not governed by the schema or an extension (or are part of the backend) should be identified and warnings should be issued for those fields. This issue has come up as part of NeurodataWithoutBorders/matnwb#245

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[t-b]

This overlaps with #942.