AWS::RDS::DBInstance needs property to initialize master user roles to enable IAM authentication

1. Title
AWS::RDS::DBInstance-InitialMasterUserRoles

2. Scope of request
New resource attribute
3. Expected behavior
To use RDS IAM authentication, one has to `GRANT` the `rds_iam` role to the RDS master user. To automate this using CloudFormation, one has to write a VPC lambda for the sole purpose of logging into the RDS instance using the master password, and running
```
GRANT rds_iam TO masteruser
```
This is a lot of complexity for such a simple operation. It would be great if there was a way to assign the initial master roles in CloudFormation, so that one could do:
```
FooDB:
Type: AWS::RDS::DBInstance
Properties:
...
InitialMasterUserRoles:
- rds_iam
...
```
5. Links
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/UsingWithRDS.IAMDBAuth.html

6. Category tag
DB

Start with the AWS RDS IAM database authentication documentation linked in the issue and the AWS::RDS::DBInstance property example. Determine how InitialMasterUserRoles should represent the requested roles and how it should interact with the master user. Done means CloudFormation can configure the initial rds_iam role without a separate VPC Lambda.

Written by the indexing model from the issue text.