crypto: reduce memory usage of SignFinal

The fixed-size buffer on the stack is unnecessary and way too large
for most applications. This change removes it and allocates the
required memory directly instead of copying into heap later.

PR-URL: https://github.com/nodejs/node/pull/23427


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>