Add option to send ORIGIN frame

This proposes adding an option to the h2::server::Builder config such that it sends the ORIGIN frame to connections.

• A method origin is added to the h2::server::Builder` struct.
  • We need to decide on the method argument. How should a user provide the origin-set?
  • The specification says the origin must ASCII, so we could just ask for a list of strings.
  • Or, we could force the user to provide a list of (Scheme, Authority) tuples, since we have those types in the http crate.
  • I'm not convinced which way is better myself.
• A new file would be added, src/frame/origin.rs.
  • Implement encoding of the ORIGIN header according the RFC.
  • We don't need decoding for now.
• Update the src/proto/connection.rs file such that Connection has an Option<Origin>, and if set, is written after the initial SETTINGS frame.

Start by reading h2::server::Builder and src/proto/connection.rs to understand configuration and the initial SETTINGS write. Then inspect the RFC requirements for ORIGIN encoding and the existing frame implementations before deciding how the origin-set argument should be represented. Done means the builder can configure an optional ORIGIN frame, src/frame/origin.rs encodes it, and the connection writes it after SETTINGS.