Add stage_url to release post item schema

Why is this change being made?

Some stages require a custom stage URL because it cannot be inferred from the stage name. Adding it to the schema as an optional field will allow it to be used in new release post items.

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what