Reuse existing NATS connection

[S7evinK]

If using external NATS, we opened unnecessary connections. This now re-uses existing connections.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 67.45%. Comparing base (b9abbf7) to head (3e14451).

Files	Patch %	Lines
setup/jetstream/nats.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3345      +/-   ##
==========================================
+ Coverage   67.42%   67.45%   +0.02%     
==========================================
  Files         521      521              
  Lines       47027    47030       +3     
==========================================
+ Hits        31708    31723      +15     
+ Misses      11355    11345      -10     
+ Partials     3964     3962       -2     

Flag	Coverage Δ
unittests	52.58% <0.00%> (+<0.01%)	⬆️

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