Allow HeaderName to be const-initialized

[caspervonb]

Use case

Provide constants of known header names in the header module without introducing any allocations.

use async_nats::header::{self, HeaderMap};

fn main() -> Result<(), String> {
  let headers = HeaderMap::new();
  headers.insert(header::NATS_STREAM, "foo".parse()?);
}

Proposed change

• Add representation that can be initialized with const for known headers.
• Change constants in in the header module to be const HeaderNames.

Who benefits from the change(s)?

Everyone

Alternative Approaches

No response