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>

pkg/model/job_context.go

@@ -0,0 +1,12 @@
+package model
+
+type JobContext struct {
+	Status    string `json:"status"`
+	Container struct {
+		ID      string `json:"id"`
+		Network string `json:"network"`
+	} `json:"container"`
+	Services map[string]struct {
+		ID string `json:"id"`
+	} `json:"services"`
+}