Minimum Support for Appending References in Hdmf-Zarr

[mavaylon1]

Motivation

What was the reasoning behind this change? Please explain the changes briefly.

Add support on the HDMF side for #1125

How to test the behavior?

Show how to reproduce the new behavior (can be a bug fix or a new feature)

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.89%. Comparing base (50aad2f) to head (7d57787).
Report is 24 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1157   +/-   ##
=======================================
  Coverage   88.89%   88.89%           
=======================================
  Files          45       45           
  Lines        9833     9834    +1     
  Branches     2794     2794           
=======================================
+ Hits         8741     8742    +1     
  Misses        776      776           
  Partials      316      316           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[mavaylon1]

Review Notes:
We have full test coverage already with this update. This is the minimum needed to have hdmf-zarr working for appending a Dataset of References. The proxy idea discussed in #1135 will be the full idea. This is being split up so Alessio can get started asap.

[stephprince]

Looks good to me.

A general question - should the same support for HDMFDataset and ZarrArray be added to extend_data?