Improve rootless container Function debugging

### What problem are you facing?

We've heard from a few folks that it's hard to determine what's going wrong when their Composition Functions don't work. This is an open-ended issue to capture ideas to improve the situation.

### How could Crossplane help solve your problem?

Some things to start with:

* [ ] Document how to use `xfn run` and/or `docker run` to debug a Composition Function in isolation.
* [ ] Have Crossplane or xfn print the `FunctionIO` objects it sees to debug logs.
* [ ] Improve the error message returned when the OCI runtime can't run the function binary for some reason (e.g. wrong architecture)