Needs a header for libcrun

crun expose a static archive(or even shared library if you want) for user, but lacks a good header file to use it. If I want to use libcrun in my project. I have to add crun functions declarations by hand.

Maybe we can clean `container.h` and `error.h` to make it usable as public header and install them to `$(PREFIX)/include/crun` for user.

Review container.h and error.h to determine which declarations are suitable for public use. Then inspect the library installation setup for how headers could be placed under $(PREFIX)/include/crun. Done means libcrun users can include the public headers without redeclaring functions.

Written by the indexing model from the issue text.