Add wrapper to store and retrieve values in context

[ccremer]

Summary

• Adds VariableContext, AddToContext, ValueFromContext functions

Usage example:

go-command-pipeline/context_test.go

Lines 58 to 70 in d0821c2

	ctx := VariableContext(context.Background())
	p := NewPipeline().WithSteps(
	NewStepFromFunc("store value", func(ctx context.Context) error {
	AddToContext(ctx, "key", "value")
	return nil
	}),
	NewStepFromFunc("retrieve value", func(ctx context.Context) error {
	value, _ := ValueFromContext(ctx, "key")
	fmt.Println(value)
	return nil
	}),
	)
	p.RunWithContext(ctx)

Checklist

• Categorize the PR by setting a good title and adding one of the labels:
  fix, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog
• Update documentation.
• Update tests.