Introduce pipeline events

[kminehart]

Example usage of events in a Pipeline:

	sw.When(
		pipeline.GitCommitEvent(pipeline.GitCommitFilters{
			Branch: pipeline.StringFilter("main"),
		}),
		pipeline.GitTagEvent(pipeline.GitTagFilters{
			Name: pipeline.GlobFilter("v*"),
		}),
	)

This doesn't have to be final and can be expanded in the future. :)

[kminehart]

@dsotirakis I'm going to go ahead and merge this but feel free to make any comments on the code or whatever and I will address them in my next PR.