[ElastiCache] createUser should map engine to be lowercase

[jflim]

In boto3, I created two Elasticache Redis users and noticed that the engine value for users get stored in lowercase. May need to adapt moto to store the passed values in lowercase.

Create with lowercase

import boto3
client = boto3.client('elasticache')
client.create_user(UserId="user1", UserName="user1", Engine = "redis", AccessString = " on ~* +@all", AuthenticationMode = { 'Type': 'iam' },)

result:

{'UserId': 'user1', 'UserName': 'user1', 'Status': 'active', 'Engine': 'redis', ...}

Create with upper case

client.create_user(UserId="user2", UserName="user2", Engine = "Redis", AccessString = " on ~* +@all", AuthenticationMode = { 'Type': 'iam' },)

result:

{'UserId': 'user2', 'UserName': 'user2', 'Status': 'active', 'Engine': 'redis', ...}

Looking at the Elasticache User API docs, the values accepted are 'valkey' or 'redis'

[armichaud]

This will be resolved by #8539