Using Winget REST api to check for duplicate packages prior to submission

[ryfu-msft]

Description of the new feature / enhancement

In winget-create, at various stages we can check to see if a user is trying to submit duplicates to the repo:

First they put in a URL, we can check if the URL is already present in the repo
Next we download the file, we can check if the hash is already present
Next they specify a packageId, we can check if the packageId is already present

These aren't necessarily hard fails, maybe just warnings.
We can also add some of this logic to the validation pipeline

Once the REST api is deployed in production, we can call it to make these checks once we add nice endpoints that let us run these queries quickly.

[ryfu-msft]

Keeping this issue to track #34 as it has a more refined resolution plan.