[terraform] make aws rds instance class configurable for self-hosting

Summary:
This makes the rds instance size configurable. We decided that db.m6g.large was likely too large and expensive for a default setup.

Unless specified, this makes db.t4g.medium the default.

Depends on D12687

Test Plan: terraform apply. RDS instance works with keyserver as before

Reviewers: bartek, varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12688