src: small modification to NgHeader

This is separated out of the QUIC PR. It is not specific to QUIC but
provides a new base class that is used there as an abstraction of
the actual implementation. This is a purely internal implementation
detail that has no outward functional changes (so no need for tests)

Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/33289


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: David Carlier <devnexen@gmail.com>