deps: cherry-pick eb306f463e from nghttp2 upstream

Original commit message:

  lib: add nghttp2_rcbuf_is_static()

  Add a `nghttp2_rcbuf_is_static()` method to tell whether a rcbuf
  is statically allocated.

  This can be useful for language bindings that wish to avoid
  creating duplicate strings for these buffers; concretely, I am
  planning to use this in the Node HTTP/2 module that is being
  introduced.

Ref: https://github.com/nghttp2/nghttp2/commit/eb306f463efea645de611de2bff0a8f823deb048
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>