chore(IP): add TrustedPlatform constant for Fly.io.

[ab]

Also add some more detail to the docs for how to use TrustedPlatform.

https://fly.io/docs/reference/runtime-environment/#fly-client-ip

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3dc1cd6) 99.21% compared to head (b1ef88d) 99.36%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3839      +/-   ##
==========================================
+ Coverage   99.21%   99.36%   +0.14%     
==========================================
  Files          42       43       +1     
  Lines        3182     2674     -508     
==========================================
- Hits         3157     2657     -500     
+ Misses         17        9       -8     
  Partials        8        8              

Flag	Coverage Δ
	?
-tags "sonic avx"	99.35% <100.00%> (?)
-tags go_json	99.35% <100.00%> (?)
-tags nomsgpack	99.35% <100.00%> (?)
go-1.18	99.28% <100.00%> (+0.16%)	⬆️
go-1.19	99.36% <100.00%> (+0.14%)	⬆️
go-1.20	99.36% <100.00%> (+0.14%)	⬆️
go-1.21	99.36% <100.00%> (+0.14%)	⬆️
macos-latest	99.36% <100.00%> (+0.14%)	⬆️
ubuntu-latest	99.36% <100.00%> (+0.14%)	⬆️

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