pkcs7: support EC-RDSA/streebog in SignerInfo

Allow using EC-RDSA/streebog in pkcs7 certificates in a similar way
to how it's done in the x509 parser.

This is needed e.g. for loading kernel modules signed with EC-RDSA.

Signed-off-by: Elvira Khabirova <e.khabirova@omp.ru>
Reviewed-by: Vitaly Chikunov <vt@altlinux.org>
Reviewed-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>