Rename plugins to match XRootD convention

[bbockelm]

Per email from upstream, the preferred naming of the plugins would be:

• libXrdOssHttp
• libXrdOssS3

I still entertain the idea of upstreaming the entirety of the plugins, meaning we'd eventually need to tackle the renaming.

• Update CMake to output the above library names by default
• Add an option for backward compatibility names, enabling to on.
• Coordinate with downstream packagers to handle the transition appropriately at OS version boundaries.

I see no problem with having -compat packages as long as needed (and keeping the existing naming scheme for EPEL9 and before, as required by the compatibility rules for that platform). Just want to start the process before EPEL 10 is released.