Avoid loading libopenfhe_julia_jll when using a custom library

[ArseniyKholod]

I tried out your suggestion from #46. It works fine on windows and linux, with and without custom library. I tried out to find in the internet whether it's a good programming practice to load package optionally, I didn't find any cons of such approach. I think it's better than apply code formatting like was done hpsc-lab/openfhe-julia#35 (comment)). So I would suggest to merge this changes. :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e573ff5) to head (3150fdc).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #50   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           36        36           
=========================================
  Hits            36        36           

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

[sloede]

LGTM, thanks! I only suggest to add a comment such that in the future we are not wondering why we chose this particular implementation.