Validate field names match DCM

[ghost]

This PR has 2 purposes:

1. Add ability to retrieve DCM from central, and validate that messages sent by device match this DCM

2. Propose a new way to organize code in repo:

azext_iot/
|--dps
|--central
  |-- providers
  |-- services
|--hub
...
|-- common
|-- utility

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Thank you for contributing to the IoT extension!

This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines

• If introducing new functionality or modified behavior, are they backed by unit and integration tests?
• In the same context as above are command names and their parameter definitions accurate? Do help docs have sufficient content?
• Have all unit and integration tests passed locally? i.e. pytest <project root> -vv
• Have static checks passed using the .pylintrc and .flake8 rules? Look at the CI scripts for example usage.

[ghost]

Tests passing