[ElastiCache] Add support for AuthenticationMode for create_user

[jflim]

Fixes #8473

[codecov]

Codecov Report

Attention: Patch coverage is 97.56098% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.63%. Comparing base (4c1321a) to head (b0cffab).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
moto/elasticache/responses.py	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8494      +/-   ##
==========================================
- Coverage   92.63%   92.63%   -0.01%     
==========================================
  Files        1224     1224              
  Lines      105925   105960      +35     
==========================================
+ Hits        98123    98153      +30     
- Misses       7802     7807       +5     

Flag	Coverage Δ
servertests	27.75% <36.58%> (+<0.01%)	⬆️
unittests	92.60% <97.56%> (-0.01%)	⬇️

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[bblommers]

LGTM - thank you @jflim!