(ec2): Support UserData interface in MultipartUserData

[ddneilson]

Simply, provide a way-forward whereby attempting to use the add*Command methods inherited from UserData do not throw exceptions. Rather, invoking the methods on MultipartUserData passes the invocation through to a designated UserData object.

Use Case

CDK construct libraries (like https://github.com/aws/aws-rfdk) allow the customer to provide a UserData object into an L3 construct. That L3 construct may use the provided UserData object in the creation of Instances, AutoScaling Groups, etc, but also use methods on the UserData such as addCommands(), addS3DownloadCommand(), etc to set up initialization required by the L3 construct.

Having these methods on the MultipartUserData throw exceptions makes it awkward at best to integrate the class with these libraries. Without this change to MultipartUserData these libraries would be required to accept both a UserData and MultipartUserData property to support MultipartUserData -- commands would be added to the UserData, but the MultipartUserData would be used to actually render the Instance/ASG's -- and enforce, somehow, that the given UserData is a part in the given MultipartUserData. It's all very awkward.

Proposed Solution

I have a candidate PR prepped, and will be posting it momentarily.

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

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This is a 🚀 Feature Request

[rix0rrr]

Good idea!

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