Task/rfr 1188 handle upload conflict

[hb0]

The issues were:

• missing index on google collection, multiple attachments metadata were allowed to be stored
• no handling when a file already exists

This PR:

• creates the missing indices for measurement and attachment metadata documents
• identifies when the collector tries to store an already stored file => inform client that file already exists

I checked manually that:

• In this instance, both copies of the uploaded file were equally large and contain the number of bytes expected (metadata.uploadLength): Both in the metadata uploadLength and the google cloud storage object linked in the metadata have the same and expected size, so we do not have an issue with "inclomplete uploads".
• The client interprets "CONFLICT" as "UPLOAD_SUCCESSFUL" and continues with the next upload.

See code for more details.

After this PR was merged

• TODO: Remove duplicate documents before we deploy this to Staging (which tries to enable the unique key indices) @hb0
• Then deployed to Staging, which enables unique indices @hb0
• Then try to upload the remaining files with the Pixel 7 and see if upload works now @muthenberg

[hb0]

pre-reviewed