group step attributes not fully supported

[JonCubed]

Problem

The Buildkite group step spec documents several optional attributes that are not currently serialised into the generated pipeline YAML (see also #170 for key):

Attribute	Status
depends_on	Silently dropped — field exists on Step but not propagated to Group during marshalling
if	Silently dropped — same root cause
notify	Silently dropped — same root cause
allow_dependency_failure	Not supported — no field exists on the Step struct

Proposed fix

Silently dropped fields (depends_on, if, notify): Add each to the Group struct and propagate from the Step in MarshalYAML, the same approach used for key in #169.

Missing fields (allow_dependency_failure): Add to the Step struct, then add to Group and propagate in MarshalYAML.