Add operation function support

[negz]

What problem are you facing?

See similar function-sdk-go PR: crossplane/function-sdk-go#212

How could this Function help solve your problem?

We should at least bump the protos and release a new version of this SDK. We may want to add some helpers for working with ops too if appropriate.

[negz]

Once this is released I'd like to prioritize getting a release of https://github.com/crossplane-contrib/function-python, which I think'll be a useful tool for operation functions. Essentially running a snippet of Python code whenever an Operation runs.