Publish JSON schema for activity configuration

I've seen from errors I've made while writing activity configurations that the provided activity configuration is validated against a JSON schema. Having the schema available would be useful for understanding the structure of the configurations, for times when it's hard to find details in the narrative documentation at http://docs.binary.ninja/dev/workflows.html.

Additionally, if the schema were checked into this repository it would be possible to leverage tools such as cargo typify to generate strongly-typed representations of the configuration. I have my own hand-rolled version of this at https://github.com/bdash/bn-bdash-extras/blob/dev/src/activity.rs because I'd rather lean on autocomplete to get the configuration structure right than have to re-read the documentation again.

Start with the activity configuration details in the workflows documentation at docs.binary.ninja/dev/workflows.html and locate the schema currently used for validation. Check how it can be represented in this repository and confirm that the published schema supports tools such as cargo typify; done means contributors can find and use the schema alongside the existing documentation.