Proposal : Extend DownloadFile to allow custom HTTP Headers

Problem statement:
The current implementation of DownloadFile does not expose any mechanism to add headers to the outgoing request. This prevents things like authenticated downloads.

The mechanics require to accomplish:

Add a parameter named `Headers` to the existing `DownloadFile` that would allow for adding additional HTTP Headers to the request.
```xml

```
Use a semi-colon separated list for multiple headers:

```xml

```