Move generic skeymgmt param decoder to .inc file

Move generic skeymgmt param decoder to .inc file

The Perl-generated code for the generic symmetric key import parameter decoder
is extracted from `generic.c.in` into a new `generic.inc.in` template.

The main `generic.c` file now includes the generated `generic.inc` file. This
change separates the auto-generated code from the handwritten C code,
improving modularity and readability.

Signed-off-by: Simo Sorce <simo@redhat.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/28838)