Support public Elasticache subnets

[hongyiweiwu01]

Is your request related to a new offering from AWS?

No,

Is your request related to a problem? Please describe.

For some of our dev environments we'd like to have direct public access to the ElastiCache clusters, which seems to require deploying them in public subnets.

Describe the solution you'd like.

We might be able to replicate the logic for public Redshift and public RDS clusters.

Describe alternatives you've considered.

Additional context

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