Update smithy-build to use NodeMapper

[mtdowling]

• Update smithy-build to use NodeMapper

  The NodeMapper is now used to load smithy-build.json configuration
  files, reducing the boilerplate needed to deserialize it (and the
  error-prone need to manually update the deserialization code as things
  are added to the POJOs). The #toNode methods and methods were removed
  from the build/model/* classes for the same reasons.
  
  An explicit POJO that defines the format of smithy-build-info.json files
  was added, and the NodeMapper is now used to serialize it. This should
  give the format a well-defined structure and allow other tools to
  properly deserialize it too.
  
  Finally, a ConfigurableSmithyBuildPlugin abstract class was added that
  makes it easier to add NodeMapper deserialization into a plugin specific
  configuration POJO. Most plugins will ultimately extend from this class
  and get deserialization for free.
  

• Adds support for more build transform options

  This commit updates smithy-build so that model transformations accept
  an object of configuration values rather than just a list of strings.
  This makes transformers far more powerful and configurable. In order to
  maintain backward compatibility with existing smithy-build.json files,
  an array of strings provided for a transformer is automatically
  transformed into `{"__args": [X]}` where "X" is the original array of
  arguments.
  ```
  
  

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[kstich]

This seems like a pretty rare case to run in to, but calling out that we're removing this without having deprecated it in 0.9.x.

[mtdowling]

I'll add a PR to master to emit a warning.