[Savings] Add services to list Flexible and Fixed products

[alphayax]

Add 2 new services related to the "Savings"

• ListSavingsFixedAndActivityProductsService
  • https://binance-docs.github.io/apidocs/spot/en/#get-fixed-and-activity-project-list-user_data
• ListSavingsFlexibleProductsService
  • https://binance-docs.github.io/apidocs/spot/en/#get-flexible-product-list-user_data

[codecov]

Codecov Report

Merging #312 (31d2bc3) into master (aea3128) will increase coverage by 0.42%.
The diff coverage is 91.11%.

@@            Coverage Diff             @@
##           master     #312      +/-   ##
==========================================
+ Coverage   73.99%   74.42%   +0.42%     
==========================================
  Files          55       56       +1     
  Lines        5269     5404     +135     
==========================================
+ Hits         3899     4022     +123     
- Misses       1009     1015       +6     
- Partials      361      367       +6     

Impacted Files	Coverage Δ
v2/savings_service.go	90.55% <90.55%> (ø)
v2/client.go	84.91% <100.00%> (+0.53%)	⬆️

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