User-defined operators

[philrz]

The PRs linked to #4152 were a first iteration on User Ops, but we recognize it has some shortcomings and loose ends that we'll likely want to address. The open issues linked to this Epic represent what we hope to address in the next iteration.

As a high-level goal, we've discussed as a team a design principle that users will hopefully be able to move any Zed pipeline from the main scope into a User Op and expect it to either work without modification or have it work with parameters passed in and referenced inside the User Op as arguments to other called operators/functions. If we find during implementation that there's exceptions to this (due to difficulty to implement, architectural limitations, etc.) then we'll need to call this out in user-facing docs.