Call da-collab back to invalidate and obtain new content if a change originated elsewhere

Description

da-collab is reached via a service binding if configured or otherwise through the address specified in the DA_COLLAB env var.

Note that while with this PR test coverage is going up, as src/routes/source.js is now covered, the computed coverage is down as this has now touched some dependencies which are not (yet) covered. This is a drawback of the way coverage was computed.

Related Issue

Fixes #28

Motivation and Context

It allows the editor to be updated when a change is made on the backend storage. This is also used when restoring a version from the UI.

How Has This Been Tested?

Unit tests added
Also tested locally with da-live and da-collab

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[X] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

[X] I have signed the Adobe Open Source CLA.
[X] My code follows the code style of this project.
[ ] My change requires a change to the documentation.
[ ] I have updated the documentation accordingly.
[X] I have read the CONTRIBUTING document.
[X] I have added tests to cover my changes.
[X] All new and existing tests passed.

Codecov Report

Attention: Patch coverage is 88.88889% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 60.55%. Comparing base (844658a) to head (23abfe9).

Files	Patch %	Lines
src/routes/source.js	88.88%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #29 +/- ## ============================================ - Coverage 100.00% 60.55% -39.45% ============================================ Files 4 13 +9 Lines 221 829 +608 ============================================ + Hits 221 502 +281 - Misses 0 327 +327 ```

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adobe / da-admin