Support connection secrets in `crossplane beta render`

### What problem are you facing?

Compositions can be used to produce XR connection secrets (aka connection details), typically by deriving them somehow from composed resources. `crossplane beta render` doesn't currently support surfacing the XR connection details returned by the Function pipeline. It also doesn't support passing in mocked connection details for composed resources (which are often the source from which XR connection details are derived).

### How could Crossplane help solve your problem?

We could support:

* Passing in mocked "observed" composed resource connection details.
* Printing out XR connection details.

One thing we'll need to consider is the input/output format. Would Kubernetes Secrets suffice? Crossplane has alpha support for other secret stores (e.g. HashiCorp Vault) but maybe Secrets are a good enough general representation.