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 <markus.wolf@new-work.se>

Co-authored-by: Markus Wolf <markus.wolf@new-work.se>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

act/model/step_result.go

@@ -0,0 +1,43 @@
+package model
+
+import "fmt"
+
+type stepStatus int
+
+const (
+	StepStatusSuccess stepStatus = iota
+	StepStatusFailure
+)
+
+var stepStatusStrings = [...]string{
+	"success",
+	"failure",
+}
+
+func (s stepStatus) MarshalText() ([]byte, error) {
+	return []byte(s.String()), nil
+}
+
+func (s *stepStatus) UnmarshalText(b []byte) error {
+	str := string(b)
+	for i, name := range stepStatusStrings {
+		if name == str {
+			*s = stepStatus(i)
+			return nil
+		}
+	}
+	return fmt.Errorf("invalid step status %q", str)
+}
+
+func (s stepStatus) String() string {
+	if int(s) >= len(stepStatusStrings) {
+		return ""
+	}
+	return stepStatusStrings[s]
+}
+
+type StepResult struct {
+	Outputs    map[string]string `json:"outputs"`
+	Conclusion stepStatus        `json:"conclusion"`
+	Outcome    stepStatus        `json:"outcome"`
+}