Add extension for QueryCollection

[justinyoo]

This PR is to add two extension methods to deal with querystring, which is for .NET 5 isolated worker.

NOTE: This feature will be deprecated when the core isolated worker introduces the same capability.

In in-proc worker, we're able to use req.Query to get the IQueryCollection instance. As there's no equivalent in the out-of-proc worker, this extension will do the same, with slightly different naming.

public static async Task<HttpResponseData> Run(
    [HttpTrigger(AuthorizationLevel.Function, "GET")] HttpRequestData req,
    FunctionContext executionContext)
{
    // To get the whole IQueryCollection
    var queries = req.Queries();

    // To get the specific query parameter value.
    var value = (string) req.Query("key");

    ...
}