Changes NewClient definition and removes bloated interface

[kislerdm]

What changed

• Removed Client interfaces
• Removed variadic variables
• Added Config as the main argument of NewClient to configure SDK
• Minor refactoring:
  • errors moved to error.go
  • mock renamed to mockhttp

Why do we need it

• Simplification: no interfaces bloating
• Ease of stubbing/mocking

[codecov]

Codecov Report

Merging #32 (74559cc) into master (d1b56b2) will decrease coverage by 0.39%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
- Coverage   93.48%   93.10%   -0.39%     
==========================================
  Files           2        3       +1     
  Lines         430      406      -24     
==========================================
- Hits          402      378      -24     
  Misses         21       21              
  Partials        7        7              

Files	Coverage Δ
error.go	100.00% <100.00%> (ø)
mockhttp.go	76.31% <ø> (ø)
sdk.go	96.62% <100.00%> (-0.56%)	⬇️

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