Improve validation of metadata documents

[davemfish]

Since our metadata documents are designed with the idea that people will edit them directly in text editors, we'll need to do more to validate these documents, so that they can be read by geometamaker or other systems.

Pydantic could be useful to do type-checking for all the dataclasses, since their attributes are already defined with types hints.

User Stories:

1. I edited a metadata document in my text editor. I want to be sure it is valid before archiving it along with the dataset.

This user may not be a Python user, so perhaps a CLI for validation is warranted.