Add Summary() for ITransformer

**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