index: Convert get_index_data() fn to async

[Turbo87]

This reduces the amount of spawn_blocking() calls in the index sync background jobs, since the queries are now handled by diesel-async :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.69%. Comparing base (59303c8) to head (161c003).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9652      +/-   ##
==========================================
- Coverage   88.69%   88.69%   -0.01%     
==========================================
  Files         288      288              
  Lines       29371    29369       -2     
==========================================
- Hits        26052    26048       -4     
- Misses       3319     3321       +2     

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