feat: add replication mode flag (v3)

[osaxma]

I am just splitting #121 (draft) in smaller PRs to push the easy pieces out of the way until #118 (simple query execution) is decided upon.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (46c3ff8) 86.71% compared to head (e050683) 86.72%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
+ Coverage   86.71%   86.72%   +0.01%     
==========================================
  Files          29       29              
  Lines        2453     2455       +2     
==========================================
+ Hits         2127     2129       +2     
  Misses        326      326              

Files Changed	Coverage Δ
lib/postgres_v3_experimental.dart	51.16% <ø> (ø)
lib/src/v3/connection.dart	87.50% <100.00%> (+0.08%)	⬆️

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