Add AccountSmsSendersEndpoint and AccountSmsSender resource

[tomasDostalDS]

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[codecov]

Codecov Report

Merging #176 (7c4c44f) into 2.x (cb73176) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                2.x     #176      +/-   ##
============================================
+ Coverage     97.41%   97.42%   +0.01%     
- Complexity      426      428       +2     
============================================
  Files            66       67       +1     
  Lines          1122     1127       +5     
============================================
+ Hits           1093     1098       +5     
  Misses           29       29              

Files	Coverage Δ
src/Endpoint/AccountEndpoint.php	100.00% <100.00%> (ø)
src/Endpoint/AccountSmsSendersEndpoint.php	100.00% <100.00%> (ø)

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