main: adapt for a multicall binary, then move ostree_ext CLI to it

[lucab]

Right now rpm-ostree is re-exporting a subset of ostree-ext CLI entries, specifically only the container bits:

rpm-ostree/rust/src/main.rs

Lines 41 to 42 in 11c3070

	"ex-container" => {
	return rpmostree_rust::container::entrypoint(&args_borrowed).await;

In the context of ostreedev/ostree#2480, we should tweak rpm-ostree into a multicall binary and map the whole ostree-ext surface, by dispatching based on argv[0].
That would also free the rpm-ostree ex-container verb for things like #3276, if needed.