runc {create,run}: --config is missing

crun has added --config option as an ability to specify name other than config.json for the spec file in https://github.com/containers/crun/pull/310, and later changed its semantics a little, making it relative to the cwd (rather than --bundle argument), and allowing paths rather than just filenames (in https://github.com/containers/crun/pull/461).

I guess we should follow the suite.

Alas, in runc it's not possible to have exec, update, or restore in a container created with --config, because runc exec requires and reads config.json. It is different in crun, as its exec and update do not rely on config.json, using a smaller config written during creation instead (see https://github.com/opencontainers/runc/issues/3180).

Thus, if we want to fix it (and I think we do -- feature parity is important), we need to fix #3180 first.

Start by reading the linked crun pull requests and runc issue #3180, which the issue identifies as a prerequisite. Trace how create/run handle the config file and how exec, update, and restore read container configuration. Done means runc supports --config for create/run and those later operations work for containers created with a non-default config path.