Tasks to control Dev Proxy from tasks.json

[garrytrinder]

We can create custom task definitions so developers can integrate Dev Proxy into their development workflows.

https://code.visualstudio.com/api/extension-guides/task-provider

Sample: https://github.com/microsoft/vscode-extension-samples/tree/main/task-provider-sample

[garrytrinder]

There is an issue with shutting down proxy which will be released in the next beta, so we can include this in the pre-release for the next version of the toolkit