Preserve connection custom extensions in SSL_set_SSL_CTX()

Preserve connection custom extensions in SSL_set_SSL_CTX()

The SSL_set_SSL_CTX() function is used to switch SSL contexts for
the given SSL object.  If contexts differ, this includes updating
a cert structure with custom extensions from the new context.  This
however overwrites connection custom extensions previously set on
top of inherited from the old context.

The fix is to preserve connection custom extensions using a newly
introduced flag SSL_EXT_FLAG_CONN in custom_ext_copy_conn().
Similar to custom_ext_copy(), it is a no-op if there are no custom
extensions to copy.

The only such consumer is ossl_quic_tls_configure() used to set the
"quic_transport_parameters" extension.  Before this change, context
switch resulted in transport parameters not being sent due to the
missing extension.

Initially reported at https://github.com/nginx/nginx/issues/711

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27706)

(cherry picked from commit 403ba31a02e47d37070036529966d5a94d98c6fd)