New crypto provider via OpenSSL library

[Cropi]

This new ossl crypto provider encapsulates the ability to use log encryption in the omfile module. It provides the same functionality as gcry, additionaly:

• Algorithm are not hardcoded. We simply provide everything what can be fetched via the EVP_CIPHER_fetch() call. The same applies to the mode parameter.
• It does not add extra dependency to a new package. Distro maintainers who are already using openssl do not need add anything in particular. By default, if rsyslog is compiled with --enable-openssl, the ossl crypto provider will also be included in both omfile and also rscryutil.
• Side-channel vulnerability in libgcrypt - the Marvin Attack
  https://lists.gnupg.org/pipermail/gcrypt-devel/2024-March/005607.html (see the whole thread)
  Some might be worried about other side-channel vulnerabilities against it, like the
  Minerva Attack[1], and the Raccoon Attack[2], given the way they handled one of the oldest, well known, and
  most straightforward side-channel attacks. They did downgrade their threat model as a result of Marvin too: https://gnupg.org/documentation/security.html. I don't see it as a proper way to deal with potential threats.

[1] https://minerva.crocs.fi.muni.cz/
[2] https://raccoon-attack.com/

Default behavior is preserved. I've tested a few different scenarios and the two crypto providers can work in tandem, but I don't suggest to mix them.
In order to put it into action, add cry.provider="ossl" to an omfile action. Generate some syslog messages, and finally use the the rscryutil with --lib=ossl. The only difference if altering the algorithm/mode then you must do it in one step, e.g.:

• ./tools/rscryutil -k keyfile -l ossl -a AES-256-CBC encryptedfile for ossl
• ./tools/rscryutil -k keyfile -l gcry -a AES256 -m CBC encryptedfile for gcry

The documentation is currently missing, I will create it if the patch is accepted. Thanks.