Make "source" a standard step filter

[moensch]

So far the only filter we have for steps is branches. We find ourselves often writing pipelines where we do different things depending on whether they're running as a schedule or via webhook or via ui. Today, we always do dynamic pipelines. Though it would be awesome if we didn't have to do that but instead use something like this in a step:

steps:
 - label: foo
   command: something
   sources: 'ui|schedule'

Or sources: '!ui'

[moensch]

I see this is now available as part of conditionals.