Allow for pre-build webhooks to be executed

[opanitch]

Use case

For monorepo integrations such as Azure DevOps, where no native messaging is supported, the `when` section is useless for helping to quickly skip builds that don't apply to the code change that triggered the build.
Our current workflow is
- AZDO webhook trigger "event.pull-request.created"
- workflow triggered/Prepare Machine
- Fetch App Sources
- restore cache
- install SDK
- get PR info from AZDO/set repo status policy to "pending"
- check PR diff against workflow project
- if match, continue with workflow, otherwise, skip all steps to publish
- set repo status policy to "success".
This takes up to 2.75 minutes w/ cache, when a build using `when` can be as short as 1 minute.  This adds up when each PR will trigger 6 builds - one for each app and package in the monorepo.

Idea

Allow for a "pre-build" step that allows the User to do similar things to the `when` condition/changeset properties