[MemoryDB] Adding memorydb User APIs

[rondodev]

This PR Implements the MemoryDB user management operations

• create_user
• describe_users
• update_user
• delete_user

[codecov]

Codecov Report

❌ Patch coverage is 96.26168% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.24%. Comparing base (115c0a4) to head (926e095).

Files with missing lines	Patch %	Lines
moto/memorydb/models.py	94.66%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #10128    +/-   ##
========================================
  Coverage   93.24%   93.24%            
========================================
  Files        1326     1326            
  Lines      120998   121104   +106     
========================================
+ Hits       112826   112928   +102     
- Misses       8172     8176     +4     

Flag	Coverage Δ
servertests	27.93% <0.00%> (-0.03%)	⬇️
unittests	93.22% <96.26%> (+<0.01%)	⬆️

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