Add Summary() for ITransformer

[LittleLittleCloud]

Describe the solution you'd like
An extension Summary() method for ITransformer that returns a string that contains information like the hyper-parameter/Transformer name if ITransformer is a single Transformer, and returns a summary of pipeline if ITransformer is ITransformeChain

Additional context
The information can be useful for debugging purposes, and also help simplify the code generator in ModelBuilder, which currently relies on a manul-maintained wrapper for all transformers to pass such information

[michaelgsharp]

@LittleLittleCloud I'll add this to our "Future" milestone, but if you are planning on getting it done for the next major release please change it to the 4.0 milestone.