fix(s3): Move the service construction to s3 service

[mantissahz]

Which issue(s) this PR fixes:

Issue # longhorn/longhorn#5840

What this PR does / why we need it:

Refactor: Move the backup store s3 service construction to the service constructor.

Special notes for your reviewer:

Additional documentation or context

Full regression test: https://ci.longhorn.io/job/private/job/longhorn-tests-regression/6585/

[innobead]

@mantissahz Please resolve the conflict, and then we can merge this PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 18.24%. Comparing base (3bce6e6) to head (0ee398c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #164   +/-   ##
=======================================
  Coverage   18.24%   18.24%           
=======================================
  Files           3        3           
  Lines         433      433           
=======================================
  Hits           79       79           
  Misses        347      347           
  Partials        7        7           

Flag	Coverage Δ
unittests	18.24% <ø> (ø)

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