Allow setting authentication in `DownloadableFile` message

[tpaschalis]

This issue is spurred by a discussion in https://github.com/open-telemetry/opamp-go/issues/196.

In some cases, it might be useful to have the server include authentication parameters along the DownloadableFile in one way or the other.

Proposal

The proposed way is to add a new headers Headers field, so that can happen via Authentication headers and provide some flexibility into what authentication method is actually used.

Alternatives considered

One alternative considered would be to accept a userinfo section in the download_url (like http://username:password@example.com/), but that is only good for Basic Auth so it locks us out of using alternative authentication methods, while some newer HTTP Semantic RFC9110 advocate for its deprecation so if might not be the best long-term solution

[tpaschalis]

I also meant to cc @phanidevavarapu who opened the original issue on the opam-go repo