Add Model Target Web API client support

[adamtheturtle]

Summary

Add public sync and async clients for the Vuforia Model Target Web API.

The API uses OAuth2 client credentials and an asynchronous dataset-generation workflow: create a standard or advanced Model Target dataset, poll its status, download the generated zip, and delete the dataset. The OpenAPI reference documents the endpoint schemas and error responses.

vws-python-mock already implements this workflow, including authentication and representative success and failure responses, so the client can be tested through real HTTP transports without bespoke test doubles.

Suggested scope

• Add synchronous and asynchronous Model Target service clients, following the existing VWS / AsyncVWS conventions.
• Authenticate with OAuth2 client credentials and manage the returned bearer token.
• Support standard and advanced dataset creation.
• Support dataset status retrieval and a wait/poll helper.
• Support dataset zip download and dataset deletion.
• Add typed request/response structures and status enums where they improve the public interface.
• Parse the Model Target API error envelope into useful public exceptions, including OAuth2/JWT authentication failures and validation errors.
• Preserve configurable base URLs and transport injection so the clients work with MockVWS.

Acceptance criteria

• Public synchronous and asynchronous APIs cover the complete create → poll → download → delete lifecycle.
• Tests exercise both clients through vws-python-mock for standard and advanced datasets.
• Tests cover representative authentication, validation, unknown-dataset, processing, and download failure responses.
• Public API documentation, examples, and a news fragment are included.