Add support of soft and external links (not resolved for now)

[loichuder]

This is more or less #95 brought up to date

Makes use of h5grove to resolve entities and links.

While already practical for the link resolution, my endgoal in introducing the h5grove dependency is to use it to encode responses, in particular data in binary, in a future PR. This would remove the simplejson dep and add explicit support for NaN and Infinity making #97 and #98 obsolete.

[github-actions]

👈 Launch a binder notebook on branch jupyterlab/jupyterlab-hdf5/h5grove-responses