filemanager: link up filemanager with postgres database manager

[mmalenic]

The filemanager should utilize the postgres manager's database creation and credential logic to create a filemanager database. This involves linking up the filemanager deployment code with the postgres manager and generating/obtaining database credentials within the Lambda function. Ideally, DbAuthType.RDS_IAM should be used for IAM authentication.

It doesn't look like the Rust sdk supports a generate_rds_iam_token function which means tokens must generated manually. See awslabs/aws-sdk-rust#951.