mrc-5219: Add new endpoint for returning the path to download result

[r-ash]

This PR builds on #471 and should be merged after that

This PR will

• Add a new endpoint to return the path and some metadata to the web app. We want this so that kotlin can form a download URL which it can pass to the front end for retrieving the the data in a way that means we can use the native browser download progress bar. See the epic ticket for more context

[codecov]

Codecov Report

Attention: Patch coverage is 97.36842% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 98.46%. Comparing base (7e3ef21) to head (1cb8aaa).
Report is 6 commits behind head on master.

❗ Current head 1cb8aaa differs from pull request most recent head 5fbf9d5. Consider uploading reports for the commit 5fbf9d5 to get more accurate results

Files	Patch %	Lines
R/endpoints.R	96.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #505      +/-   ##
==========================================
- Coverage   98.49%   98.46%   -0.03%     
==========================================
  Files          29       29              
  Lines        2389     2414      +25     
==========================================
+ Hits         2353     2377      +24     
- Misses         36       37       +1     

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