http2: minor refactor of passing headers to JS

- Make `ExternalString::New` return a `MaybeLocal`. Failing is
  left up to the caller.
- Allow creating internalized strings for short header names
  to reduce memory consumption and increase performance.
- Use persistent storage for statically allocated header names.

PR-URL: https://github.com/nodejs/node/pull/14808


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>