Add viewkey only benchmark

[zancas]

• reorganize LightClient impl blocks into constructors and other
• alphabetize each impl block
• remove integration_tests feature gate
• unify test utils
• export and use fvks for a client
• rename ClientManager to ClientBuilder

[codecov-commenter]

Codecov Report

Patch coverage: 47.96% and project coverage change: +0.14 🎉

Comparison is base (991104e) 60.33% compared to head (e52cb9f) 60.48%.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #411      +/-   ##
==========================================
+ Coverage   60.33%   60.48%   +0.14%     
==========================================
  Files          34       34              
  Lines        5252     5276      +24     
==========================================
+ Hits         3169     3191      +22     
- Misses       2083     2085       +2     

Impacted Files	Coverage Δ
zingocli/tests/darkside/tests.rs	85.45% <ø> (ø)
zingolib/src/commands.rs	0.00% <ø> (ø)
zingolib/src/wallet/keys.rs	57.14% <ø> (ø)
zingolib/src/lightclient.rs	62.14% <36.31%> (-0.10%)	⬇️
zingo-testutils/src/lib.rs	86.80% <96.55%> (+0.95%)	⬆️
zingocli/tests/integration_tests.rs	84.84% <100.00%> (+1.24%)	⬆️

... and 4 files with indirect coverage changes

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[fluidvanadium]

ok

[fluidvanadium]

this is good