feat(amazon-sp): validate credentials on save

[s-aga-r]

Validate credentials on save with Amazon. In case of any wrong credential(s), an error will be thrown with a description.

Example:

[codecov]

Codecov Report

Merging #176 (0e02721) into develop (f8ac70b) will increase coverage by 0.27%.
The diff coverage is 38.09%.

@@             Coverage Diff             @@
##           develop     #176      +/-   ##
===========================================
+ Coverage    40.93%   41.20%   +0.27%     
===========================================
  Files           62       62              
  Lines         4124     4138      +14     
===========================================
+ Hits          1688     1705      +17     
+ Misses        2436     2433       -3     

Impacted Files	Coverage Δ
...on/doctype/amazon_sp_api_settings/amazon_sp_api.py	60.98% <0.00%> (+4.00%)	⬆️
...e/amazon_sp_api_settings/amazon_sp_api_settings.py	36.73% <33.33%> (-0.23%)	⬇️
...octype/amazon_sp_api_settings/amazon_repository.py	64.30% <87.50%> (+0.61%)	⬆️