Header providers for multipart request

[mrandrewsmith]

The problem

builder.add(provider: provider) was ignored for multipart requests

What is done

• Added header providers for multipart request
• Created tests for new functionality

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.26%. Comparing base (1aaa23c) to head (fda636e).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
NodeKit/NodeKit/Chains/ServiceChainProvider.swift	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
+ Coverage   92.23%   93.26%   +1.03%     
==========================================
  Files          85       85              
  Lines        1326     1826     +500     
==========================================
+ Hits         1223     1703     +480     
- Misses        103      123      +20     

Flag	Coverage Δ
tests	93.26% <81.81%> (+1.03%)	⬆️

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The base branch was changed.