Add CMS_NO_SIGNING_TIME flag to CMS_sign(), CMS_add1_signer()

Add CMS_NO_SIGNING_TIME flag to CMS_sign(), CMS_add1_signer()

Previously there was no way to create a CMS SignedData signature without a
signing time attribute, because CMS_SignerInfo_sign added it unconditionally.
However, there is a use case (PAdES signatures) where this attribute is not
allowed, so this commit introduces a new flag to the CMS API that causes this
attribute to be omitted at signing time.

Also add -no_signing_time option to cms command.

Fixes #15777

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15783)