refactor: export and move shared contexts into pkg/model (#931)

This commit moves the githubContext, jobContext and stepResult structs
from the runner package to the model package in preparation for #908
because the expression.go file lives in the runner package and would
introduce cyclic dependencies with the exprparser package.

Co-authored-by: Markus Wolf <[email protected]>

Co-authored-by: Markus Wolf <[email protected]>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

pkg/runner/runner.go

@@ -188,7 +188,7 @@ func (runner *runnerImpl) newRunContext(run *model.Run, matrix map[string]interf
 		Config:      runner.config,
 		Run:         run,
 		EventJSON:   runner.eventJSON,
-		StepResults: make(map[string]*stepResult),
+		StepResults: make(map[string]*model.StepResult),
 		Matrix:      matrix,
 	}
 	rc.ExprEval = rc.NewExpressionEvaluator()