WIP: RDS: add engineVersion attribute

[RaitoBezarius]

This adds the engine version attribute (e.g. PostgreSQL 12, etc.)

This PR only require I make the attribute optional so that it is backward compatible.

[tewfik-ghariani]

I believe it should also be added to :

nixops-aws/nixops_aws/resources/types/ec2_rds_dbinstance.py

Line 5 in 9690edc

class Ec2RdsDbinstanceOptions(ResourceOptions):

Also, I would suggest to adapt the boto3 port work done per #120 and add this particular change ^^