[keyserver] Support external blobs in uploads table

Summary:
Part of ENG-3530.

• Added possibility of storing blobHolder in the extra column of the uploads table.
• Having this value requires the content column to be empty (for consistency - data is stored either on keyserver or Blob service).
• Fetching uploads from keyserver url should return resource_unavailable error if the blobHolder is set.

Depends on D7617

Test Plan:

• Repeated test plan from two previous diffs, this time updated the DB columns directly instead of artifically hardcoding blob URIs.
• Also tried to fetch external upload with http://KEYSERVER_ADDR/uploads/{id}/{secret} - ensured that resource_unavailable error is thrown.
• DB Insertion will be tested in further diffs.

Reviewers: atul, marcin, inka, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7618