Add botan2 support back for ubuntu, rockylinux #262.

[hks2002]

@marty1885
Please consider add botan2 support back. There are too many OS pkg still using botan2.
https://pkgs.org/search/?q=botan

[marty1885]

I noticed the lack of Botan3 support in distros too. Thanks for bring up. However my dev rigs all runs Arch which only has Botan 3. It'll be quite difficult to make sure Botan 2 is working properly when there's no one actively using it.

I can add the support if there's active users of a Botan 2 backend and they are ok with reporting related bugs.

[hks2002]

In my windows, botan3 is working, I add a way to prepare the dependencies build by source with Cmake FetchContent in my code, I tried to build botan3 in source for ubuntu, but the building is faild due to 8G mem is not enough for GCC compile. This also cause we can't build botan in github actions.

please add code:

#ifdef USE_BOTAN2
#ifdef USE_BOTAN3

in BotanTLSProvider.cc to handle different botan version.

[marty1885]

It's not that easy. Botan2 and 3 have different interfaces and error handling. So much so and in security critical sections that I much prefer to separate the backends for Botan 2/3.