[terraform] initial Redis terraform config

Summary:
Initial Redis configuration with elasticache serverless.

Depends on D12457

Test Plan:
Ran on personal aws account. Successfully connected remotely with redis-cli.
Plan on testing with keyserver before landing

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12458