HTTP client

[9seconds]

This PR creates httransform own HTTP client. This is required until fasthttp implements streaming of bodies back to the client.

[codecov-io]

Codecov Report

Merging #1 into master will decrease coverage by 5.88%.
The diff coverage is 71.76%.

@@            Coverage Diff            @@
##           master      #1      +/-   ##
=========================================
- Coverage   87.59%   81.7%   -5.89%     
=========================================
  Files          14      21       +7     
  Lines         774    1159     +385     
=========================================
+ Hits          678     947     +269     
- Misses         66     162      +96     
- Partials       30      50      +20

Impacted Files	Coverage Δ
client/pools.go	100% <100%> (ø)
http_clients.go	36.84% <27.27%> (-35.16%)	⬇️
client/dialer.go	28.57% <28.57%> (ø)
client/client.go	45.76% <45.76%> (ø)
executor.go	75.75% <66.66%> (+4.32%)	⬆️
client/pooled_dialer.go	74.07% <74.07%> (ø)
client/readers.go	83.58% <83.58%> (ø)
client/simple_dialer.go	90% <90%> (ø)
client/conns.go	95% <95%> (ø)
... and 7 more

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