Lock down model constructors

[nwithan8]

Description

• Mark constructors for all models as "internal" to prevent end-users from making instances directly
  • var address = new Address() \\ bad
  • var address = await myClient.Address.Retrieve() \\ good
• Update unit tests for F# and VB to account for redesign

Testing

• All unit tests pass

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)