Validating assets calls github repeatedly

When calling `dandischema.metadata.validate` on n assets, n requests are made to github to fetch the schema. This makes validating assets take significantly longer than it should. The request also has no default timeout, meaning a call to `validate` can hang indefinitely.

https://github.com/dandi/dandi-schema/blob/d34658cc24c0e3c0c3a88e92bcd29b158241448e/dandischema/metadata.py#L184-L187

Can `dandi-schema` be modified to avoid relying on the network for validation? Either by bundling the schemas from `dandi/schema` into package data, allowing the caller of `validate` to pass a schema directly, or some other means?

FWIW this problem appears to exist with `migrate` as well.