feat: add ckanext-zarr submodule

Description

As it says on the tin - adds a new custom extension for the ZaRR project.

ckanext-zarr is added as a submodule (.gitmodules) and an editable install into the container (Pipfile).

A new Pipfile.lock has been generated using pipenv version 2024.0.1

relates fjelltopp/fjelltopp-infrastructure#240

[ ] The Jira ticket for this issue has been updated to "Ready to Review" or equivalent.
[x] I have developed these changes in discussion with the appropriate project manager.
[x] My code follows the general Fjelltopp documentation (see Confluence).
[ ] I have made corresponding changes to the Fjelltopp documentation (see Confluence).
[x] I have rebased this branch with master.
[x] New dependency changes have been committed.
[ ] I have added automated tests that prove my fix is effective or that my feature works.
[x] New and existing tests pass locally with my changes.
[x] My changes generate no new warnings.
[x] I have performed a self-review of my own code.
[x] I have assigned at least one reviewer.
[ ] I have assigned at least one label to this PR: "patch", "minor", "major".