RFE: introduce plugin API version + load-time checking

[pmatilai]

As our plugins become public there needs to be some ways to check they're compiled for a compatible version of rpm. Many will get that enforcement through rpm soname but technically there's no requirement for a plugin to link to librpm* and .. better safe than sorry with mysterious bugreports from people having plugins for some subtly incompatible version loaded.

Split into a separate item from #1536