[Feature Request] Enable using HTTP/2 for discovery endpoints

[prochnowc]

Is your feature request related to a problem? Please describe.
I want to use HTTP/2 for discovery endpoints.

Describe the solution you'd like
The HttpDocumentRetriever should have an option to allow specifying the HTTP version and version policy.

Describe alternatives you've considered
Implementing my own HttpDocumentRetriever.

[prochnowc]

@pmaytak Is there anything I can do, so that the issue gets some attention?

[pmaytak]

@prochnowc We'll try to take this in this week's release.