[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