Add Composition diffing

[jcogilvie]

What problem are you facing?

We'd like to be able to see what changes will be made in the cluster when we change a composition.

How could Crossplane help solve your problem?

• rename crossplane-diff diff to crossplane-diff xr
• add crossplane-diff comp

the diff comp command should take a composition yaml as its argument, and if an existing composition matches it on the cluster, it should download the requisite files to run a Render pass against the updated Composition, and then show a diff of any XRs (and their downstream MRs) that would be affected by the change.