Add doctl serverless trigger support (for scheduled functions)

[joshuaauerbachwatson]

This PR adds support (still hidden) for a possible future release of doctl serverless that includes support for scheduled functions.

At the moment, this change should not be merged (I am labelling it a draft) because there is a known pending change to how communication with the backend will be handled (we know the code will not be released in its current form).

At the same time, the present code works and can be evaluated so I am creating the PR to make it a little more visible.

More detail to be added when this is ready for a pre-merge review.