worker: Implement async enqueue() fn

[Turbo87]

This PR implements an async variant of the BackgroundJob::enqueue() fn, that can be used inside other async fns, without the need to use spawn_blocking().

I'm not a fan of the (temporary) code duplication, but due to the diesel type madness I wasn't successful in extracting the common parts into shared fns.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 160 lines in your changes missing coverage. Please review.

Project coverage is 88.67%. Comparing base (bbe1922) to head (a909a46).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
src/admin/enqueue_job.rs	0.00%	72 Missing ⚠️
crates/crates_io_worker/src/background_job.rs	0.00%	51 Missing ⚠️
src/admin/delete_crate.rs	0.00%	37 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9632      +/-   ##
==========================================
- Coverage   88.87%   88.67%   -0.20%     
==========================================
  Files         288      288              
  Lines       29264    29326      +62     
==========================================
- Hits        26007    26006       -1     
- Misses       3257     3320      +63     

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