Feature: Service Packages / Registration

[tombuildsstuff]

This commit introduces the Service Registration and Service Builder

The AzureRM Provider is using Service Packages to allow us to break up the codebase into smaller pieces. A key component of this is the Service Registration functionality - which allows each Service to return a selection of Meta Data relevant for that Service.

The Service Builder is a new bit of functionality which can be used to compose Service Packages together, to be able to obtain a canonical list of Data Sources and Resource supported by a Terraform Provider.

[hashicorp-cla]

All committers have signed the CLA.