Add db_subnet_group to restore_dbinstance_from_point_in_time

[reversefold]

So you can use point-in-time restores with VPC.

[toastdriven]

Overall, this looks like a good change. However, to be merged, it'll need tests. If you can write a test covering the addition, that would be great. Otherwise, it'll need to wait for a core dev to have time to add one.

The correct place would be within https://github.com/boto/boto/blob/develop/tests/unit/rds/test_connection.py. Just needs to cover the new flag.

[reversefold]

I don't see any tests for restore_dbinstance_from_point_in_time at all so I can't simply add this parameter to the existing tests. It seems a bit disingenuous to require unit tests for simple changes which add missing parameters when there are no unit tests covering the code yet as it is.

[reversefold]

I've added minimal tests to cover the default parameters and the subnet group name using the AWS example response.

[danielgtaylor]

This LGTM now that tests have been added 👍