support for ro-crate metadata

[RO-Crate](https://www.researchobject.org/ro-crate/) is a metadata convention within academia to describe datasets in a sidecar approach. Would be nice to have import and export options from MCF

(Some) Academic repositories pick up the embedded RO-Crate metadata via a [signposting convention](https://www.researchobject.org/ro-crate/specification/1.2/data-entities#retrieving-an-ro-crate)

Some deposits including RO-Crate metadata:

- https://doi.org/10.4121/21304308.v1 (Notice the RO-Crate button in sidebar)
- https://doi.org/10.5281/zenodo.5753092

Start by reading the linked RO-Crate specification and reviewing the example deposits to understand the metadata convention and signposting approach. Then identify pygeometa's existing import and export entry points; done means RO-Crate metadata can be imported from and exported to the sidecar format.

Written by the indexing model from the issue text.