yashim/feat: add requestDataTransformer middleware functionality

[yashim-deriv]

• Add requestDataTransformer() functionality to DerivAPIBasic
• This function allows user to attach a function to modify request_data on both send() and subscribe() method
• If no requestDataTransformer() is passed, it will just pass through the original request_data
• Updated tests as well

[codecov-commenter]

Codecov Report

Merging #198 (f9eee0c) into master (70627aa) will increase coverage by 0.05%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
+ Coverage   58.70%   58.76%   +0.05%     
==========================================
  Files          39       39              
  Lines        1441     1443       +2     
  Branches      286      288       +2     
==========================================
+ Hits          846      848       +2     
  Misses        594      594              
  Partials        1        1              

Impacted Files	Coverage Δ
src/deriv_api/DerivAPIBasic.js	85.20% <100.00%> (+0.17%)	:arrow_up: