(ec2): Cannot attach Volume to Instance

ec2.Instance only currently supports attaching volumes with type BlockDeviceVolume.
See
https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-ec2.Instance.html#blockdevices
and
https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-ec2.BlockDevice.html

### Use Case

CloudFormation supports attaching Volumes created separately from the Instance. It would be nice to have the same support for the L2 CDK classes.

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-instance.html#cfn-ec2-instance-volumes

### Proposed Solution
I believe this would involve adding a new property, `volumes` to [Instance](https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-ec2.Instance.html#blockdevices) of a type that maps a device name to [Volume](https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-ec2.Volume.html).
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-mount-point.html

### Other

* [ ] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

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This is a :rocket: Feature Request

Start with the EC2 Instance, BlockDeviceVolume, and Volume APIs, then compare them with CloudFormation's volumes and mount-point properties. Done means Instance supports attaching separately created Volumes by device name, matching the requested L2 behavior.

Written by the indexing model from the issue text.