[terraform] Move blob scheduler code to aws_iam.tf and rename scheduler to task_scheduler

Summary:
This moves the scheduler role and policy to a shared location in aws_iam.tf for usage by other scheduled tasks. And renames scheduler to task_scheduler

Depends on D11126

Test Plan: terraform fmt and plan

Reviewers: varun

Reviewed By: varun

Subscribers: tomek, ashoat

Differential Revision: https://phab.comm.dev/D11161